diff --git a/css/leptonChrome.css b/css/leptonChrome.css
index 947d856..9120f7e 100644
--- a/css/leptonChrome.css
+++ b/css/leptonChrome.css
@@ -59,7 +59,7 @@
     padding-inline-end: var(--arrowpanel-menuitem-padding-inline) !important;
   }
   /*= First visible tab margin at maximized #332 ===============================*/
-  :root[tabsintitlebar="true"][sizemode="maximized"] #TabsToolbar {
+  :root:is([tabsintitlebar="true"], [customtitlebar="true"])[sizemode="maximized"] #TabsToolbar {
     margin-left: -1px;
   }
   /*= Disabled menu background color ===========================================*/
@@ -183,18 +183,18 @@
     align-items: center !important;
   }
   /*= Inactive transparent #882 ================================================*/
-  :root[tabsintitlebar] #titlebar:-moz-window-inactive,
-  :root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive,
-  :root[tabsintitlebar] #TabsToolbar:-moz-window-inactive {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive,
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive,
+  :root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive {
     opacity: 1 !important;
     color: color-mix(in srgb, currentColor calc(var(--inactive-titlebar-opacity, 0.75) * 100%), transparent) !important;
     --inactive-toolbarbutton-icon-fill-opacity: calc(
       var(--toolbarbutton-icon-fill-opacity, 1) * var(--inactive-titlebar-opacity, 0.75)
     );
   }
-  :root[tabsintitlebar] #titlebar:-moz-window-inactive .toolbarbutton-icon,
-  :root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive .toolbarbutton-icon,
-  :root[tabsintitlebar] #TabsToolbar:-moz-window-inactive .toolbarbutton-icon {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive .toolbarbutton-icon,
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive .toolbarbutton-icon,
+  :root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive .toolbarbutton-icon {
     fill-opacity: var(--inactive-toolbarbutton-icon-fill-opacity, 1);
   }
   /*= Tabpreviw transparent #890 ===============================================*/
@@ -223,19 +223,19 @@
 @supports -moz-bool-pref("userChrome.compatibility.os") {
   /*= Windows 10 - Top border of accent color at ESR #358 ======================*/
   @media (-moz-platform: windows) {
-    :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+    :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
       border-top: 0.5px solid rgb(47, 47, 47) !important;
     }
-    :root[sizemode="normal"][tabsintitlebar]:-moz-window-inactive #navigator-toolbox {
+    :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]):-moz-window-inactive #navigator-toolbox {
       border-top-color: rgb(57, 57, 57) !important;
     }
   }
   @media (-moz-platform: windows) and (-moz-windows-accent-color-in-titlebar) {
-    :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+    :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
       border-top-color: ActiveBorder !important;
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+      :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
         border-top-color: -moz-accent-color !important;
       }
     }
@@ -300,13 +300,13 @@
   /*= Titlebar Container Size at maximized #384 ================================*/
   @media (-moz-platform: windows) {
     @supports not -moz-bool-pref("userChrome.compatibility.os.windows_maximized") {
-      :root[tabsintitlebar] #titlebar {
+      :root:is([tabsintitlebar], [customtitlebar]) #titlebar {
         /* -moz-default-appearance: -moz-window-titlebar */
         appearance: none !important;
       }
     }
     @supports -moz-bool-pref("userChrome.compatibility.os.windows_maximized") {
-      :root[tabsintitlebar][sizemode="maximized"] #titlebar {
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #titlebar {
         appearance: none !important;
         padding-top: 8px;
       }
@@ -331,14 +331,14 @@
   /*= Common - URL Bar focus color =============================================*/
   @media (-moz-windows-accent-color-in-titlebar), (-moz-gtk-csd-available) {
     /* URL Bar */
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
       --focus-outline-color: AccentColor !important;
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
       #urlbar[open]
       > #urlbar-background,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
       #urlbar[open]
       > #urlbar-background {
       border-color: color-mix(
@@ -348,14 +348,14 @@
       ) !important; /* Like: --toolbar-field-focus-border-color */
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
         --focus-outline-color: -moz-accent-color !important;
       }
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
         #urlbar[open]
         > #urlbar-background,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
         #urlbar[open]
         > #urlbar-background {
         border-color: color-mix(
@@ -620,8 +620,11 @@
   @media (-moz-platform: windows) {
     @supports -moz-bool-pref("userChrome.theme.system_default") {
       @media (-moz-platform: windows) and (-moz-windows-accent-color-in-titlebar) {
-        :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(
+            :not([lwtheme]),
+            :not(:-moz-lwtheme)
+          ),
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
           --toolbarseparator-color: color-mix(in srgb, currentColor 20%, transparent) !important; /* As default */
         }
       }
@@ -797,27 +800,35 @@
   /*= Windows10 - Titlebar accent color ========================================*/
   @media (-moz-windows-accent-color-in-titlebar) {
     /* Tab Bar */
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .titlebar-color,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .titlebar-color {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      .titlebar-color,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      .titlebar-color {
       color: AccentColorText;
       background-color: AccentColor;
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .titlebar-color,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .titlebar-color {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        .titlebar-color,
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        .titlebar-color {
         color: -moz-accent-color-foreground;
         background-color: -moz-accent-color;
       }
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .toolbar-items,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .toolbar-items {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      .toolbar-items,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      .toolbar-items {
       --toolbarbutton-icon-fill: currentColor;
       --toolbarbutton-hover-background: color-mix(in srgb, AccentColorText 10%, transparent);
       --toolbarbutton-active-background: color-mix(in srgb, AccentColorText 15%, transparent);
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .toolbar-items,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .toolbar-items {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        .toolbar-items,
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        .toolbar-items {
         --toolbarbutton-hover-background: color-mix(in srgb, -moz-accent-color-foreground 10%, transparent);
         --toolbarbutton-active-background: color-mix(in srgb, -moz-accent-color-foreground 15%, transparent);
       }
@@ -4087,13 +4098,13 @@
     --uc-space-left-tabbar: 8px; /* If the option is not specified, it is equivalent to 0px. */
   }
   @supports not -moz-bool-pref("userChrome.padding.first_tab.always") {
-    :root:not([tabsintitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
+    :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
     :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs:not([positionpinnedtabs]) {
       padding-inline-start: var(--uc-space-left-tabbar) !important;
     }
   }
   @supports -moz-bool-pref("userChrome.padding.first_tab.always") {
-    :root:not([tabsintitlebar]) #tabbrowser-tabs,
+    :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs,
     :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs {
       padding-inline-start: calc(var(--tab-overflow-pinned-tabs-width, 0px) + var(--uc-space-left-tabbar)) !important;
     }
@@ -4254,21 +4265,21 @@
     --uc-space-above-tabbar: 8px; /* If the option is not specified, it is equivalent to 0px. */
   }
   @media (-moz-platform: windows) {
-    :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
       height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar) - var(--tabs-navbar-shadow-size));
     }
     @supports -moz-bool-pref("userChrome.padding.drag_space.maximized") {
-      :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
         height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar) - var(--tabs-navbar-shadow-size));
       }
     }
   }
   @media (-moz-gtk-csd-available) {
-    :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
       height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar));
     }
     @supports -moz-bool-pref("userChrome.padding.drag_space.maximized") {
-      :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
         height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar));
       }
     }
@@ -4387,7 +4398,7 @@
     /* Original: calc(var(--tab-border-radius) + 4px) = 8px */
     border-radius: var(--scrollbtn-border-radius, calc(var(--tab-border-radius) + 4px)) !important;
   }
-  :root[tabsintitlebar]:not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar]):not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
     height: calc(
       var(--tab-min-height) - var(--tabs-navbar-shadow-size) - 2px
     ); /* Compact: 28px, Normal: 33px, Touch: 38px */
@@ -4879,7 +4890,7 @@
         width: 100%;
         overflow: hidden;
       }
-      :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
         height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
       }
       #toolbar-menubar > .titlebar-buttonbox-container {
@@ -4992,7 +5003,7 @@
             width: 100%;
             overflow: hidden;
           }
-          :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+          :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
             height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
           }
           #toolbar-menubar > .titlebar-buttonbox-container {
@@ -5248,75 +5259,75 @@
     --uc-window-drag-space-post: 0px; /* Same as .titlebar-spacer[type="post-tabs"] */
     --uc-window-control-space: calc(var(--uc-window-control-width) + var(--uc-window-drag-space-post));
   }
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: var(--uc-title-pre-spacer, 40px);
     --uc-window-drag-space-post: var(--uc-title-post-spacer, 40px);
   }
   @media (-moz-platform: windows) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 138px;
     }
   }
   @media (-moz-gtk-csd-available) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       /* 84px is default value of linux */
     }
   }
   @media (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-maximize-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-close-button) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 28px;
     }
   }
   @media (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-close-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 56px;
     }
   }
   @media (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 84px;
     }
   }
   @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled") {
-    :root[tabsintitlebar] {
+    :root:is([tabsintitlebar], [customtitlebar]) {
       --uc-window-control-width: 72px;
     }
     :root[sizemode="fullscreen"] {
       --uc-window-control-space: 0px;
     }
   }
-  :root:is([tabsintitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar][sizemode="maximized"], [customtitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
   @supports -moz-bool-pref("userChrome.tabbar.one_liner") {
     @supports not -moz-bool-pref("userChrome.tabbar.one_liner.responsive") {
       @supports -moz-bool-pref("userChrome.tabbar.one_liner.tabbar_first") {
         @media (-moz-platform: windows) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-drag-space-pre: 0px;
           }
         }
         @media (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-drag-space-pre: 0px;
           }
         }
         @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled") {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-control-space: 0px;
           }
         }
         @media (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-control-space: 0px;
           }
         }
         @supports -moz-bool-pref("userChrome.tabbar.as_titlebar") {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
             --uc-window-control-space: 0px;
           }
         }
@@ -5324,12 +5335,12 @@
       @supports not -moz-bool-pref("userChrome.tabbar.one_liner.tabbar_first") {
         @supports not -moz-bool-pref("userChrome.tabbar.as_titlebar") {
           @media (-moz-platform: windows) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
           @media (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
@@ -5340,7 +5351,7 @@
       @media screen and (max-width: 1100px) {
         @supports (not -moz-bool-pref("userChrome.tabbar.on_bottom")) or
           (not -moz-bool-pref("userChrome.hidden.tabbar")) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-drag-space-pre: 0px;
             --uc-window-control-space: 0px;
           }
@@ -5349,27 +5360,27 @@
       @media screen and (min-width: 1100px) {
         @supports -moz-bool-pref("userChrome.tabbar.one_liner.tabbar_first") {
           @media screen and (min-width: 1100px) and (-moz-platform: windows) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-drag-space-pre: 0px;
             }
           }
           @media screen and (min-width: 1100px) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-drag-space-pre: 0px;
             }
           }
           @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled") {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
           @media screen and (min-width: 1100px) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
           @supports -moz-bool-pref("userChrome.tabbar.as_titlebar") {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
               --uc-window-control-space: 0px;
             }
           }
@@ -5377,12 +5388,12 @@
         @supports not -moz-bool-pref("userChrome.tabbar.one_liner.tabbar_first") {
           @supports not -moz-bool-pref("userChrome.tabbar.as_titlebar") {
             @media screen and (min-width: 1100px) and (-moz-platform: windows) {
-              :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+              :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
                 --uc-window-control-space: 0px;
               }
             }
             @media screen and (min-width: 1100px) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-              :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+              :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
                 --uc-window-control-space: 0px;
               }
             }
@@ -5460,11 +5471,11 @@
           }
         }
         @media (-moz-platform: windows) {
-          :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+          :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
             margin-top: calc(-1 * var(--uc-titlebar-padding));
           }
-          :root[sizemode="normal"][tabsintitlebar] #titlebar,
-          :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+          :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+          :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
             appearance: none !important;
           }
           .browser-toolbar:not(.titlebar-color) {
@@ -5624,11 +5635,11 @@
             }
           }
           @media (-moz-platform: windows) {
-            :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
               margin-top: calc(-1 * var(--uc-titlebar-padding));
             }
-            :root[sizemode="normal"][tabsintitlebar] #titlebar,
-            :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+            :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
               appearance: none !important;
             }
             .browser-toolbar:not(.titlebar-color) {
@@ -5785,11 +5796,11 @@
             }
           }
           @media (-moz-platform: windows) {
-            :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
               margin-top: calc(-1 * var(--uc-titlebar-padding));
             }
-            :root[sizemode="normal"][tabsintitlebar] #titlebar,
-            :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+            :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
               appearance: none !important;
             }
             .browser-toolbar:not(.titlebar-color) {
@@ -5955,11 +5966,11 @@
             }
           }
           @media screen and (max-width: 1100px) and (-moz-platform: windows) {
-            :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
               margin-top: calc(-1 * var(--uc-titlebar-padding));
             }
-            :root[sizemode="normal"][tabsintitlebar] #titlebar,
-            :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+            :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
               appearance: none !important;
             }
             .browser-toolbar:not(.titlebar-color) {
@@ -6132,11 +6143,11 @@
               }
             }
             @media screen and (min-width: 1100px) and (-moz-platform: windows) {
-              :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
                 margin-top: calc(-1 * var(--uc-titlebar-padding));
               }
-              :root[sizemode="normal"][tabsintitlebar] #titlebar,
-              :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+              :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
                 appearance: none !important;
               }
               .browser-toolbar:not(.titlebar-color) {
@@ -6295,11 +6306,11 @@
               }
             }
             @media screen and (min-width: 1100px) and (-moz-platform: windows) {
-              :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
                 margin-top: calc(-1 * var(--uc-titlebar-padding));
               }
-              :root[sizemode="normal"][tabsintitlebar] #titlebar,
-              :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+              :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
                 appearance: none !important;
               }
               .browser-toolbar:not(.titlebar-color) {
@@ -6714,7 +6725,7 @@
 }
 /*= Tab Bar - Show only current tab ==========================================*/
 @supports -moz-bool-pref("userChrome.tabbar.as_titlebar") {
-  :root:not([tabsintitlebar="true"]) #tabbrowser-tabs,
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"]) #tabbrowser-tabs,
   #scrollbutton-up,
   #scrollbutton-down,
   .titlebar-spacer[type="pre-tabs"],
@@ -6783,8 +6794,10 @@
     }
   }
   @supports -moz-bool-pref("browser.tabs.tabmanager.enabled") {
-    :root:not([tabsintitlebar="true"]) #tabbrowser-tabs:not([overflow="true"], [hashiddentabs]) ~ #alltabs-button,
-    :root:not([tabsintitlebar="true"])
+    :root:not([tabsintitlebar="true"]):not([customtitlebar="true"])
+      #tabbrowser-tabs:not([overflow="true"], [hashiddentabs])
+      ~ #alltabs-button,
+    :root:not([tabsintitlebar="true"]):not([customtitlebar="true"])
       #tabbrowser-tabs[hasadjacentnewtabbutton]:not([overflow="true"])
       ~ #new-tab-button {
       display: flex !important;
@@ -8038,13 +8051,16 @@
     --uc-newtab-bgcolor: var(--toolbarbutton-active-background);
   }
   @media (-moz-windows-accent-color-in-titlebar) {
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) #tabs-newtab-button:hover,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      #tabs-newtab-button:hover,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      #tabs-newtab-button:hover {
       fill: color-mix(in srgb, AccentColorText 10%, transparent) !important; /* Hardcoded for compatibility */
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
       #tabs-newtab-button:hover:active,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      #tabs-newtab-button:hover:active {
       fill: color-mix(in srgb, AccentColorText 15%, transparent) !important; /* Hardcoded for compatibility */
     }
     @supports -moz-bool-pref("userChrome.tab.bottom_rounded_corner") {
@@ -8053,9 +8069,10 @@
         ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.chrome_legacy") or -moz-bool-pref(
           "userChrome.tab.bottom_rounded_corner.chrome"
         ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.edge") {
-        :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
           #tabs-newtab-button:hover,
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+          #tabs-newtab-button:hover {
           background-image: var(--uc-tab-corner-left-side-svg),
             linear-gradient(
               to left,
@@ -8064,9 +8081,9 @@
             ),
             var(--uc-tab-corner-right-side-svg);
         }
-        :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
           #tabs-newtab-button:hover:active,
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
           #tabs-newtab-button:hover:active {
           background-image: var(--uc-tab-corner-left-side-svg),
             linear-gradient(
@@ -8079,18 +8096,19 @@
       }
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
         #tabs-newtab-button:hover,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        #tabs-newtab-button:hover {
         fill: color-mix(
           in srgb,
           -moz-accent-color-foreground 10%,
           transparent
         ) !important; /* Hardcoded for compatibility */
       }
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
         #tabs-newtab-button:hover:active,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
         #tabs-newtab-button:hover:active {
         fill: color-mix(
           in srgb,
@@ -8104,9 +8122,13 @@
           ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.chrome_legacy") or -moz-bool-pref(
             "userChrome.tab.bottom_rounded_corner.chrome"
           ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.edge") {
-          :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+          :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(
+              :not([lwtheme]),
+              :not(:-moz-lwtheme)
+            )
             #tabs-newtab-button:hover,
-          :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+          :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+            #tabs-newtab-button:hover {
             background-image: var(--uc-tab-corner-left-side-svg),
               linear-gradient(
                 to left,
@@ -8115,9 +8137,12 @@
               ),
               var(--uc-tab-corner-right-side-svg);
           }
-          :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+          :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(
+              :not([lwtheme]),
+              :not(:-moz-lwtheme)
+            )
             #tabs-newtab-button:hover:active,
-          :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+          :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
             #tabs-newtab-button:hover:active {
             background-image: var(--uc-tab-corner-left-side-svg),
               linear-gradient(
@@ -9970,7 +9995,7 @@
     }
     @media (-moz-gtk-csd-available) {
       /* Fix transparent background */
-      :root[tabsintitlebar][sizemode="fullscreen"]:not([inDOMFullscreen="true"]):is(
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"]:not([inDOMFullscreen="true"]):is(
           :not([lwtheme]),
           :not(:-moz-lwtheme)
         )
@@ -10224,24 +10249,24 @@
     }
   }
   @supports selector(:has(a)) {
-    :root:not([tabsintitlebar]):has(#toolbar-menubar) {
+    :root:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar) {
       --uc-menubar-height: var(--uc-menubar-height-default);
     }
     @media (-moz-gtk-csd-available) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) + 2 * var(--tab-block-margin));
       }
     }
     @media (-moz-platform: windows) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) - var(--tabs-navbar-shadow-size));
       }
     }
-    :root[tabsintitlebar]:has(#toolbar-menubar[autohide="false"]) {
+    :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="false"]) {
       --uc-menubar-height: var(--uc-titlebar-buttonbox-height);
     }
-    :root:not([tabsintitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]),
-    :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"][inactive="true"]) {
+    :root:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]),
+    :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]) {
       --uc-menubar-height: 0px;
     }
     #navigator-toolbox:has(#PersonalToolbar[collapsed="true"]) {
@@ -14064,7 +14089,7 @@
     padding-inline-end: var(--arrowpanel-menuitem-padding-inline) !important;
   }
   /*= First visible tab margin at maximized #332 ===============================*/
-  :root[tabsintitlebar="true"][sizemode="maximized"] #TabsToolbar {
+  :root:is([tabsintitlebar="true"], [customtitlebar="true"])[sizemode="maximized"] #TabsToolbar {
     margin-left: -1px;
   }
   /*= Disabled menu background color ===========================================*/
@@ -14220,18 +14245,18 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.compatibility.theme") {
-  :root[tabsintitlebar] #titlebar:-moz-window-inactive,
-  :root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive,
-  :root[tabsintitlebar] #TabsToolbar:-moz-window-inactive {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive,
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive,
+  :root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive {
     opacity: 1 !important;
     color: color-mix(in srgb, currentColor calc(var(--inactive-titlebar-opacity, 0.75) * 100%), transparent) !important;
     --inactive-toolbarbutton-icon-fill-opacity: calc(
       var(--toolbarbutton-icon-fill-opacity, 1) * var(--inactive-titlebar-opacity, 0.75)
     );
   }
-  :root[tabsintitlebar] #titlebar:-moz-window-inactive .toolbarbutton-icon,
-  :root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive .toolbarbutton-icon,
-  :root[tabsintitlebar] #TabsToolbar:-moz-window-inactive .toolbarbutton-icon {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive .toolbarbutton-icon,
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive .toolbarbutton-icon,
+  :root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive .toolbarbutton-icon {
     fill-opacity: var(--inactive-toolbarbutton-icon-fill-opacity, 1);
   }
 }
@@ -14269,20 +14294,20 @@
   /*= Windows - Classic Theme #734 =============================================*/
 }
 @media (-moz-bool-pref: "userChrome.compatibility.os") and (-moz-platform: windows) {
-  :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
     border-top: 0.5px solid rgb(47, 47, 47) !important;
   }
-  :root[sizemode="normal"][tabsintitlebar]:-moz-window-inactive #navigator-toolbox {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]):-moz-window-inactive #navigator-toolbox {
     border-top-color: rgb(57, 57, 57) !important;
   }
 }
 @media (-moz-bool-pref: "userChrome.compatibility.os") and (-moz-platform: windows) and (-moz-windows-accent-color-in-titlebar) {
-  :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
     border-top-color: ActiveBorder !important;
   }
 }
 @media (-moz-bool-pref: "userChrome.compatibility.os") and (-moz-platform: windows) and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") {
-  :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
     border-top-color: -moz-accent-color !important;
   }
 }
@@ -14340,13 +14365,13 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.compatibility.os") and (-moz-platform: windows) and (not (-moz-bool-pref: "userChrome.compatibility.os.windows_maximized")) {
-  :root[tabsintitlebar] #titlebar {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar {
     /* -moz-default-appearance: -moz-window-titlebar */
     appearance: none !important;
   }
 }
 @media (-moz-bool-pref: "userChrome.compatibility.os") and (-moz-platform: windows) and (-moz-bool-pref: "userChrome.compatibility.os.windows_maximized") {
-  :root[tabsintitlebar][sizemode="maximized"] #titlebar {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #titlebar {
     appearance: none !important;
     padding-top: 8px;
   }
@@ -14372,14 +14397,16 @@
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar),
   (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-gtk-csd-available) {
   /* URL Bar */
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
     --focus-outline-color: AccentColor !important;
   }
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
     #urlbar[open]
     > #urlbar-background,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #urlbar[open] > #urlbar-background {
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #urlbar[open]
+    > #urlbar-background {
     border-color: color-mix(
       in srgb,
       AccentColor 50%,
@@ -14389,14 +14416,16 @@
 }
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color"),
   (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-gtk-csd-available) and (-moz-bool-pref: "userChrome.compatibility.accent_color") {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
     --focus-outline-color: -moz-accent-color !important;
   }
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
     #urlbar[open]
     > #urlbar-background,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #urlbar[open] > #urlbar-background {
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #urlbar[open]
+    > #urlbar-background {
     border-color: color-mix(
       in srgb,
       -moz-accent-color 50%,
@@ -14655,8 +14684,8 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-platform: windows) and (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar) {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
     --toolbarseparator-color: color-mix(in srgb, currentColor 20%, transparent) !important; /* As default */
   }
 }
@@ -14825,30 +14854,38 @@
 }
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar) {
   /* Tab Bar */
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .titlebar-color,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .titlebar-color {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    .titlebar-color,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    .titlebar-color {
     color: AccentColorText;
     background-color: AccentColor;
   }
 }
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .titlebar-color,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .titlebar-color {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    .titlebar-color,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    .titlebar-color {
     color: -moz-accent-color-foreground;
     background-color: -moz-accent-color;
   }
 }
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar) {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .toolbar-items,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .toolbar-items {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    .toolbar-items,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    .toolbar-items {
     --toolbarbutton-icon-fill: currentColor;
     --toolbarbutton-hover-background: color-mix(in srgb, AccentColorText 10%, transparent);
     --toolbarbutton-active-background: color-mix(in srgb, AccentColorText 15%, transparent);
   }
 }
 @media (-moz-bool-pref: "userChrome.theme.system_default") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .toolbar-items,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .toolbar-items {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    .toolbar-items,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    .toolbar-items {
     --toolbarbutton-hover-background: color-mix(in srgb, -moz-accent-color-foreground 10%, transparent);
     --toolbarbutton-active-background: color-mix(in srgb, -moz-accent-color-foreground 15%, transparent);
   }
@@ -18263,13 +18300,13 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.first_tab") and (not (-moz-bool-pref: "userChrome.padding.first_tab.always")) {
-  :root:not([tabsintitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
+  :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
   :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs:not([positionpinnedtabs]) {
     padding-inline-start: var(--uc-space-left-tabbar) !important;
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.first_tab") and (-moz-bool-pref: "userChrome.padding.first_tab.always") {
-  :root:not([tabsintitlebar]) #tabbrowser-tabs,
+  :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs,
   :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs {
     padding-inline-start: calc(var(--tab-overflow-pinned-tabs-width, 0px) + var(--uc-space-left-tabbar)) !important;
   }
@@ -18442,22 +18479,22 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.drag_space") and (-moz-platform: windows) {
-  :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
     height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar) - var(--tabs-navbar-shadow-size));
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.drag_space") and (-moz-platform: windows) and (-moz-bool-pref: "userChrome.padding.drag_space.maximized") {
-  :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
     height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar) - var(--tabs-navbar-shadow-size));
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.drag_space") and (-moz-gtk-csd-available) {
-  :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
     height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar));
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.drag_space") and (-moz-gtk-csd-available) and (-moz-bool-pref: "userChrome.padding.drag_space.maximized") {
-  :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
     height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar));
   }
 }
@@ -18584,7 +18621,7 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.padding.tabbar_height") {
-  :root[tabsintitlebar]:not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar]):not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
     height: calc(
       var(--tab-min-height) - var(--tabs-navbar-shadow-size) - 2px
     ); /* Compact: 28px, Normal: 33px, Touch: 38px */
@@ -19079,7 +19116,7 @@
     width: 100%;
     overflow: hidden;
   }
-  :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
     height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
   }
   #toolbar-menubar > .titlebar-buttonbox-container {
@@ -19186,7 +19223,7 @@
     width: 100%;
     overflow: hidden;
   }
-  :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
     height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
   }
   #toolbar-menubar > .titlebar-buttonbox-container {
@@ -19466,7 +19503,7 @@
     --uc-window-drag-space-post: 0px; /* Same as .titlebar-spacer[type="post-tabs"] */
     --uc-window-control-space: calc(var(--uc-window-control-width) + var(--uc-window-drag-space-post));
   }
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: var(--uc-title-pre-spacer, 40px);
     --uc-window-drag-space-post: var(--uc-title-post-spacer, 40px);
   }
@@ -19475,7 +19512,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-platform: windows) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-width: 138px;
   }
 }
@@ -19483,7 +19520,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-gtk-csd-available),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-gtk-csd-available),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     /* 84px is default value of linux */
   }
 }
@@ -19499,7 +19536,7 @@
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-maximize-button),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-close-button) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-width: 28px;
   }
 }
@@ -19515,7 +19552,7 @@
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-close-button),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-width: 56px;
   }
 }
@@ -19523,7 +19560,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-width: 84px;
   }
 }
@@ -19531,7 +19568,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled"),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled"),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") {
-  :root[tabsintitlebar] {
+  :root:is([tabsintitlebar], [customtitlebar]) {
     --uc-window-control-width: 72px;
   }
   :root[sizemode="fullscreen"] {
@@ -19542,7 +19579,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner"),
   (-moz-bool-pref: "userChrome.hidden.tabbar"),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") {
-  :root:is([tabsintitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar][sizemode="maximized"], [customtitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
 }
@@ -19550,7 +19587,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
 }
@@ -19558,7 +19595,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
 }
@@ -19566,7 +19603,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled"),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled"),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19574,7 +19611,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19582,7 +19619,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "userChrome.tabbar.as_titlebar"),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "userChrome.tabbar.as_titlebar"),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
     --uc-window-control-space: 0px;
   }
 }
@@ -19590,7 +19627,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-platform: windows) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19598,7 +19635,7 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19610,7 +19647,7 @@
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (max-width: 1100px) and (not (-moz-bool-pref: "userChrome.hidden.tabbar")),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (max-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.on_bottom")),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (max-width: 1100px) and (not (-moz-bool-pref: "userChrome.hidden.tabbar")) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
     --uc-window-control-space: 0px;
   }
@@ -19619,7 +19656,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
 }
@@ -19627,7 +19664,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
 }
@@ -19635,7 +19672,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled"),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled"),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19643,7 +19680,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19651,7 +19688,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "userChrome.tabbar.as_titlebar"),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "userChrome.tabbar.as_titlebar"),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
     --uc-window-control-space: 0px;
   }
 }
@@ -19659,7 +19696,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-platform: windows) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19667,7 +19704,7 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0),
   screen and (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first")) and (not (-moz-bool-pref: "userChrome.tabbar.as_titlebar")) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-control-space: 0px;
   }
 }
@@ -19795,11 +19832,11 @@
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner")) and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner")) and (-moz-bool-pref: "userChrome.tabbar.on_bottom") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (not (-moz-bool-pref: "userChrome.tabbar.one_liner")) and (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-platform: windows) {
-  :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
     margin-top: calc(-1 * var(--uc-titlebar-padding));
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
@@ -20035,11 +20072,11 @@
 @media (-moz-bool-pref: "userChrome.tabbar.on_bottom") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows) {
-  :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
     margin-top: calc(-1 * var(--uc-titlebar-padding));
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
@@ -20242,11 +20279,11 @@
 @media (-moz-bool-pref: "userChrome.tabbar.on_bottom") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") and (-moz-platform: windows),
   (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive")) and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") and (-moz-platform: windows) {
-  :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
     margin-top: calc(-1 * var(--uc-titlebar-padding));
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
@@ -20479,11 +20516,11 @@
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (-moz-bool-pref: "userChrome.hidden.tabbar") and (max-width: 1100px) and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (-moz-bool-pref: "userChrome.tabbar.on_bottom") and (max-width: 1100px) and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (-moz-bool-pref: "userChrome.hidden.tabbar") and (max-width: 1100px) and (-moz-platform: windows) {
-  :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
     margin-top: calc(-1 * var(--uc-titlebar-padding));
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
@@ -20719,11 +20756,11 @@
 @media screen and (-moz-bool-pref: "userChrome.tabbar.on_bottom") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "userChrome.tabbar.one_liner.tabbar_first") and (-moz-platform: windows) {
-  :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
     margin-top: calc(-1 * var(--uc-titlebar-padding));
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
@@ -20926,11 +20963,11 @@
 @media screen and (-moz-bool-pref: "userChrome.tabbar.on_bottom") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") and (-moz-platform: windows),
   screen and (-moz-bool-pref: "userChrome.hidden.tabbar") and (not (-moz-bool-pref: "userChrome.hidden.titlebar_container")) and (-moz-bool-pref: "userChrome.tabbar.one_liner") and (-moz-bool-pref: "userChrome.tabbar.one_liner.responsive") and (min-width: 1100px) and (-moz-bool-pref: "layout.css.osx-font-smoothing.enabled") and (-moz-platform: windows) {
-  :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
     margin-top: calc(-1 * var(--uc-titlebar-padding));
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
@@ -21346,7 +21383,7 @@
 }
 /*= Tab Bar - Show only current tab ==========================================*/
 @media (-moz-bool-pref: "userChrome.tabbar.as_titlebar") {
-  :root:not([tabsintitlebar="true"]) #tabbrowser-tabs,
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"]) #tabbrowser-tabs,
   #scrollbutton-up,
   #scrollbutton-down,
   .titlebar-spacer[type="pre-tabs"],
@@ -21414,8 +21451,10 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.tabbar.as_titlebar") and (-moz-bool-pref: "browser.tabs.tabmanager.enabled") {
-  :root:not([tabsintitlebar="true"]) #tabbrowser-tabs:not([overflow="true"], [hashiddentabs]) ~ #alltabs-button,
-  :root:not([tabsintitlebar="true"])
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"])
+    #tabbrowser-tabs:not([overflow="true"], [hashiddentabs])
+    ~ #alltabs-button,
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"])
     #tabbrowser-tabs[hasadjacentnewtabbutton]:not([overflow="true"])
     ~ #new-tab-button {
     display: flex !important;
@@ -22822,13 +22861,16 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) #tabs-newtab-button:hover,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    #tabs-newtab-button:hover,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover {
     fill: color-mix(in srgb, AccentColorText 10%, transparent) !important; /* Hardcoded for compatibility */
   }
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
     #tabs-newtab-button:hover:active,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover:active {
     fill: color-mix(in srgb, AccentColorText 15%, transparent) !important; /* Hardcoded for compatibility */
   }
 }
@@ -22837,8 +22879,10 @@
   (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner.chrome_legacy"),
   (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner.chrome"),
   (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner.edge") {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) #tabs-newtab-button:hover,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    #tabs-newtab-button:hover,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover {
     background-image: var(--uc-tab-corner-left-side-svg),
       linear-gradient(
         to left,
@@ -22847,9 +22891,10 @@
       ),
       var(--uc-tab-corner-right-side-svg);
   }
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
     #tabs-newtab-button:hover:active,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover:active {
     background-image: var(--uc-tab-corner-left-side-svg),
       linear-gradient(
         to left,
@@ -22860,17 +22905,20 @@
   }
 }
 @media (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) #tabs-newtab-button:hover,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    #tabs-newtab-button:hover,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover {
     fill: color-mix(
       in srgb,
       -moz-accent-color-foreground 10%,
       transparent
     ) !important; /* Hardcoded for compatibility */
   }
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
     #tabs-newtab-button:hover:active,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover:active {
     fill: color-mix(
       in srgb,
       -moz-accent-color-foreground 15%,
@@ -22883,8 +22931,10 @@
   (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner.chrome_legacy"),
   (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner.chrome"),
   (-moz-bool-pref: "userChrome.tab.newtab_button_like_tab") and (-moz-windows-accent-color-in-titlebar) and (-moz-bool-pref: "userChrome.compatibility.accent_color") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner") and (-moz-bool-pref: "userChrome.tab.bottom_rounded_corner.edge") {
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) #tabs-newtab-button:hover,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    #tabs-newtab-button:hover,
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover {
     background-image: var(--uc-tab-corner-left-side-svg),
       linear-gradient(
         to left,
@@ -22893,9 +22943,10 @@
       ),
       var(--uc-tab-corner-right-side-svg);
   }
-  :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
     #tabs-newtab-button:hover:active,
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    #tabs-newtab-button:hover:active {
     background-image: var(--uc-tab-corner-left-side-svg),
       linear-gradient(
         to left,
@@ -24870,7 +24921,10 @@
 }
 @media (-moz-bool-pref: "userChrome.fullscreen.overlap") and (-moz-bool-pref: "browser.fullscreen.autohide") and (-moz-gtk-csd-available) {
   /* Fix transparent background */
-  :root[tabsintitlebar][sizemode="fullscreen"]:not([inDOMFullscreen="true"]):is(:not([lwtheme]), :not(:-moz-lwtheme))
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"]:not([inDOMFullscreen="true"]):is(
+      :not([lwtheme]),
+      :not(:-moz-lwtheme)
+    )
     #TabsToolbar {
     appearance: auto !important;
   }
@@ -25134,7 +25188,7 @@
   (-moz-bool-pref: "userChrome.hidden.tabbar"),
   (-moz-bool-pref: "userChrome.navbar.as_sidebar") {
   @supports selector(:has(a)) {
-    :root:not([tabsintitlebar]):has(#toolbar-menubar) {
+    :root:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar) {
       --uc-menubar-height: var(--uc-menubar-height-default);
     }
     @media (-moz-bool-pref: "userChrome.autohide.tabbar") and (-moz-gtk-csd-available),
@@ -25144,7 +25198,7 @@
       (-moz-bool-pref: "userChrome.tabbar.on_bottom.menubar_on_top") and (-moz-gtk-csd-available),
       (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-gtk-csd-available),
       (-moz-bool-pref: "userChrome.navbar.as_sidebar") and (-moz-gtk-csd-available) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) + 2 * var(--tab-block-margin));
       }
     }
@@ -25155,15 +25209,15 @@
       (-moz-bool-pref: "userChrome.tabbar.on_bottom.menubar_on_top") and (-moz-platform: windows),
       (-moz-bool-pref: "userChrome.hidden.tabbar") and (-moz-platform: windows),
       (-moz-bool-pref: "userChrome.navbar.as_sidebar") and (-moz-platform: windows) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) - var(--tabs-navbar-shadow-size));
       }
     }
-    :root[tabsintitlebar]:has(#toolbar-menubar[autohide="false"]) {
+    :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="false"]) {
       --uc-menubar-height: var(--uc-titlebar-buttonbox-height);
     }
-    :root:not([tabsintitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]),
-    :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"][inactive="true"]) {
+    :root:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]),
+    :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]) {
       --uc-menubar-height: 0px;
     }
     #navigator-toolbox:has(#PersonalToolbar[collapsed="true"]) {
diff --git a/css/leptonChromeESR.css b/css/leptonChromeESR.css
index 2783f84..6a32286 100644
--- a/css/leptonChromeESR.css
+++ b/css/leptonChromeESR.css
@@ -59,7 +59,7 @@
     padding-inline-end: var(--arrowpanel-menuitem-padding-inline) !important;
   }
   /*= First visible tab margin at maximized #332 ===============================*/
-  :root[tabsintitlebar="true"][sizemode="maximized"] #TabsToolbar {
+  :root:is([tabsintitlebar="true"], [customtitlebar="true"])[sizemode="maximized"] #TabsToolbar {
     margin-left: -1px;
   }
   /*= Disabled menu background color ===========================================*/
@@ -183,18 +183,18 @@
     align-items: center !important;
   }
   /*= Inactive transparent #882 ================================================*/
-  :root[tabsintitlebar] #titlebar:-moz-window-inactive,
-  :root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive,
-  :root[tabsintitlebar] #TabsToolbar:-moz-window-inactive {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive,
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive,
+  :root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive {
     opacity: 1 !important;
     color: color-mix(in srgb, currentColor calc(var(--inactive-titlebar-opacity, 0.75) * 100%), transparent) !important;
     --inactive-toolbarbutton-icon-fill-opacity: calc(
       var(--toolbarbutton-icon-fill-opacity, 1) * var(--inactive-titlebar-opacity, 0.75)
     );
   }
-  :root[tabsintitlebar] #titlebar:-moz-window-inactive .toolbarbutton-icon,
-  :root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive .toolbarbutton-icon,
-  :root[tabsintitlebar] #TabsToolbar:-moz-window-inactive .toolbarbutton-icon {
+  :root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive .toolbarbutton-icon,
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive .toolbarbutton-icon,
+  :root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive .toolbarbutton-icon {
     fill-opacity: var(--inactive-toolbarbutton-icon-fill-opacity, 1);
   }
   /*= Tabpreviw transparent #890 ===============================================*/
@@ -223,20 +223,20 @@
 @supports -moz-bool-pref("userChrome.compatibility.os") {
   /*= Windows 10 - Top border of accent color at ESR #358 ======================*/
   @media (-moz-os-version: windows-win10), (-moz-platform: windows-win10) {
-    :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+    :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
       border-top: 0.5px solid rgb(47, 47, 47) !important;
     }
-    :root[sizemode="normal"][tabsintitlebar]:-moz-window-inactive #navigator-toolbox {
+    :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]):-moz-window-inactive #navigator-toolbox {
       border-top-color: rgb(57, 57, 57) !important;
     }
   }
   @media (-moz-os-version: windows-win10) and (-moz-windows-accent-color-in-titlebar),
     (-moz-platform: windows-win10) and (-moz-windows-accent-color-in-titlebar) {
-    :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+    :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
       border-top-color: ActiveBorder !important;
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[sizemode="normal"][tabsintitlebar] #navigator-toolbox {
+      :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox {
         border-top-color: -moz-accent-color !important;
       }
     }
@@ -433,7 +433,7 @@
     (-moz-os-version: windows-win10),
     (-moz-platform: windows) {
     @supports not -moz-bool-pref("userChrome.compatibility.os.windows_maximized") {
-      :root[tabsintitlebar] #titlebar {
+      :root:is([tabsintitlebar], [customtitlebar]) #titlebar {
         /* -moz-default-appearance: -moz-window-titlebar */
         appearance: none !important;
       }
@@ -453,13 +453,13 @@
         (-moz-platform: windows) and (-moz-platform: windows-win7),
         (-moz-platform: windows) and (-moz-os-version: windows-win8),
         (-moz-platform: windows) and (-moz-platform: windows-win8) {
-        :root[tabsintitlebar][sizemode="maximized"] #titlebar {
+        :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #titlebar {
           padding-top: 8px;
         }
       }
     }
     @supports -moz-bool-pref("userChrome.compatibility.os.windows_maximized") {
-      :root[tabsintitlebar][sizemode="maximized"] #titlebar {
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #titlebar {
         appearance: none !important;
         padding-top: 8px;
       }
@@ -470,18 +470,18 @@
     (-moz-os-version: windows-win8) and (-moz-windows-classic),
     (-moz-os-version: windows-win10) and (-moz-windows-classic),
     (-moz-platform: windows) and (-moz-windows-classic) {
-    :root[tabsintitlebar][sizemode="normal"]:not(:-moz-lwtheme) #navigator-toolbox {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"]:not(:-moz-lwtheme) #navigator-toolbox {
       appearance: auto !important;
       -moz-default-appearance: -moz-window-titlebar !important;
     }
-    :root[tabsintitlebar][sizemode="maximized"]:not(:-moz-lwtheme) #navigator-toolbox {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"]:not(:-moz-lwtheme) #navigator-toolbox {
       appearance: auto !important;
       -moz-default-appearance: -moz-window-titlebar !important;
     }
-    :root[tabsintitlebar]:not(:-moz-lwtheme) #tabbrowser-arrowscrollbox {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-lwtheme) #tabbrowser-arrowscrollbox {
       color: -moz-dialogtext !important;
     }
-    :root[tabsintitlebar][sizemode="normal"] #navigator-toolbox:-moz-lwtheme {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #navigator-toolbox:-moz-lwtheme {
       background-image: linear-gradient(
         to bottom,
         ThreeDLightShadow 0,
@@ -512,14 +512,14 @@
   /*= Common - URL Bar focus color =============================================*/
   @media (-moz-windows-accent-color-in-titlebar), (-moz-gtk-csd-available) {
     /* URL Bar */
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
       --focus-outline-color: AccentColor !important;
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
       #urlbar[open]
       > #urlbar-background,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
       #urlbar[open]
       > #urlbar-background {
       border-color: color-mix(
@@ -529,14 +529,14 @@
       ) !important; /* Like: --toolbar-field-focus-border-color */
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
         --focus-outline-color: -moz-accent-color !important;
       }
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
         #urlbar[open]
         > #urlbar-background,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
         #urlbar[open]
         > #urlbar-background {
         border-color: color-mix(
@@ -836,8 +836,11 @@
     @supports -moz-bool-pref("userChrome.theme.system_default") {
       @media (-moz-os-version: windows-win10) and (-moz-windows-accent-color-in-titlebar),
         (-moz-platform: windows-win10) and (-moz-windows-accent-color-in-titlebar) {
-        :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)),
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(
+            :not([lwtheme]),
+            :not(:-moz-lwtheme)
+          ),
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
           --toolbarseparator-color: color-mix(in srgb, currentColor 20%, transparent) !important; /* As default */
         }
       }
@@ -1019,27 +1022,35 @@
   /*= Windows10 - Titlebar accent color ========================================*/
   @media (-moz-windows-accent-color-in-titlebar) {
     /* Tab Bar */
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .titlebar-color,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .titlebar-color {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      .titlebar-color,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      .titlebar-color {
       color: AccentColorText;
       background-color: AccentColor;
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .titlebar-color,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .titlebar-color {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        .titlebar-color,
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        .titlebar-color {
         color: -moz-accent-color-foreground;
         background-color: -moz-accent-color;
       }
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .toolbar-items,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .toolbar-items {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      .toolbar-items,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      .toolbar-items {
       --toolbarbutton-icon-fill: currentColor;
       --toolbarbutton-hover-background: color-mix(in srgb, AccentColorText 10%, transparent);
       --toolbarbutton-active-background: color-mix(in srgb, AccentColorText 15%, transparent);
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) .toolbar-items,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) .toolbar-items {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        .toolbar-items,
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        .toolbar-items {
         --toolbarbutton-hover-background: color-mix(in srgb, -moz-accent-color-foreground 10%, transparent);
         --toolbarbutton-active-background: color-mix(in srgb, -moz-accent-color-foreground 15%, transparent);
       }
@@ -4407,13 +4418,13 @@
     --uc-space-left-tabbar: 8px; /* If the option is not specified, it is equivalent to 0px. */
   }
   @supports not -moz-bool-pref("userChrome.padding.first_tab.always") {
-    :root:not([tabsintitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
+    :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
     :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs:not([positionpinnedtabs]) {
       padding-inline-start: var(--uc-space-left-tabbar) !important;
     }
   }
   @supports -moz-bool-pref("userChrome.padding.first_tab.always") {
-    :root:not([tabsintitlebar]) #tabbrowser-tabs,
+    :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs,
     :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs {
       padding-inline-start: calc(var(--tab-overflow-pinned-tabs-width, 0px) + var(--uc-space-left-tabbar)) !important;
     }
@@ -4577,11 +4588,11 @@
     --uc-space-above-tabbar: 8px; /* If the option is not specified, it is equivalent to 0px. */
   }
   @media (-moz-os-version: windows-win10), (-moz-platform: windows-win10) {
-    :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
       height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar) - var(--tabs-navbar-shadow-size));
     }
     @supports -moz-bool-pref("userChrome.padding.drag_space.maximized") {
-      :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
         height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar) - var(--tabs-navbar-shadow-size));
       }
     }
@@ -4591,11 +4602,11 @@
     (-moz-os-version: windows-win8),
     (-moz-platform: windows-win8),
     (-moz-gtk-csd-available) {
-    :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
       height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar));
     }
     @supports -moz-bool-pref("userChrome.padding.drag_space.maximized") {
-      :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
         height: calc(var(--tab-min-height) + var(--uc-space-above-tabbar));
       }
     }
@@ -4732,7 +4743,7 @@
     /* Original: calc(var(--tab-border-radius) + 4px) = 8px */
     border-radius: var(--scrollbtn-border-radius, calc(var(--tab-border-radius) + 4px)) !important;
   }
-  :root[tabsintitlebar]:not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar]):not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
     height: calc(
       var(--tab-min-height) - var(--tabs-navbar-shadow-size) - 2px
     ); /* Compact: 28px, Normal: 33px, Touch: 38px */
@@ -5241,7 +5252,7 @@
         width: 100%;
         overflow: hidden;
       }
-      :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
         height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
       }
       #toolbar-menubar > .titlebar-buttonbox-container {
@@ -5354,7 +5365,7 @@
             width: 100%;
             overflow: hidden;
           }
-          :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+          :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
             height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
           }
           #toolbar-menubar > .titlebar-buttonbox-container {
@@ -5610,7 +5621,7 @@
     --uc-window-drag-space-post: 0px; /* Same as .titlebar-spacer[type="post-tabs"] */
     --uc-window-control-space: calc(var(--uc-window-control-width) + var(--uc-window-drag-space-post));
   }
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: var(--uc-title-pre-spacer, 40px);
     --uc-window-drag-space-post: var(--uc-title-post-spacer, 40px);
   }
@@ -5618,48 +5629,48 @@
     (-moz-platform: windows-win7),
     (-moz-os-version: windows-win8),
     (-moz-platform: windows-win8) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 105px;
     }
   }
   @media (-moz-os-version: windows-win10), (-moz-platform: windows-win10) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 138px;
     }
   }
   @media (-moz-gtk-csd-available) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       /* 84px is default value of linux */
     }
   }
   @media (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-maximize-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-close-button) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 28px;
     }
   }
   @media (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-close-button),
     (-moz-gtk-csd-available) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 56px;
     }
   }
   @media (-moz-gtk-csd-available) and (-moz-gtk-csd-minimize-button) and (-moz-gtk-csd-maximize-button) and (-moz-gtk-csd-close-button) {
-    :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+    :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
       --uc-window-control-width: 84px;
     }
   }
   @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled") {
-    :root[tabsintitlebar] {
+    :root:is([tabsintitlebar], [customtitlebar]) {
       --uc-window-control-width: 72px;
     }
     :root[sizemode="fullscreen"] {
       --uc-window-control-space: 0px;
     }
   }
-  :root:is([tabsintitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar][sizemode="maximized"], [customtitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
     --uc-window-drag-space-pre: 0px;
   }
   @supports -moz-bool-pref("userChrome.tabbar.one_liner") {
@@ -5669,27 +5680,27 @@
           (-moz-os-version: windows-win8),
           (-moz-os-version: windows-win10),
           (-moz-platform: windows) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-drag-space-pre: 0px;
           }
         }
         @media (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-drag-space-pre: 0px;
           }
         }
         @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled") {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-control-space: 0px;
           }
         }
         @media (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-control-space: 0px;
           }
         }
         @supports -moz-bool-pref("userChrome.tabbar.as_titlebar") {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
             --uc-window-control-space: 0px;
           }
         }
@@ -5700,12 +5711,12 @@
             (-moz-os-version: windows-win8),
             (-moz-os-version: windows-win10),
             (-moz-platform: windows) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
           @media (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
@@ -5716,7 +5727,7 @@
       @media screen and (max-width: 1100px) {
         @supports (not -moz-bool-pref("userChrome.tabbar.on_bottom")) or
           (not -moz-bool-pref("userChrome.hidden.tabbar")) {
-          :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+          :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
             --uc-window-drag-space-pre: 0px;
             --uc-window-control-space: 0px;
           }
@@ -5728,27 +5739,27 @@
             screen and (min-width: 1100px) and (-moz-os-version: windows-win8),
             screen and (min-width: 1100px) and (-moz-os-version: windows-win10),
             screen and (min-width: 1100px) and (-moz-platform: windows) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-drag-space-pre: 0px;
             }
           }
           @media screen and (min-width: 1100px) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-drag-space-pre: 0px;
             }
           }
           @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled") {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
           @media screen and (min-width: 1100px) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement) {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
               --uc-window-control-space: 0px;
             }
           }
           @supports -moz-bool-pref("userChrome.tabbar.as_titlebar") {
-            :root:is([tabsintitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
+            :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) #tabbrowser-tabs {
               --uc-window-control-space: 0px;
             }
           }
@@ -5759,12 +5770,12 @@
               screen and (min-width: 1100px) and (-moz-os-version: windows-win8),
               screen and (min-width: 1100px) and (-moz-os-version: windows-win10),
               screen and (min-width: 1100px) and (-moz-platform: windows) {
-              :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+              :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
                 --uc-window-control-space: 0px;
               }
             }
             @media screen and (min-width: 1100px) and (-moz-gtk-csd-available) and (-moz-gtk-csd-reversed-placement: 0) {
-              :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+              :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
                 --uc-window-control-space: 0px;
               }
             }
@@ -5845,14 +5856,14 @@
           (-moz-os-version: windows-win8),
           (-moz-os-version: windows-win10),
           (-moz-platform: windows) {
-          :root[sizemode="maximized"][tabsintitlebar] {
+          :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
             --uc-titlebar-padding: 8px;
           }
-          :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+          :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
             margin-top: calc(-1 * var(--uc-titlebar-padding));
           }
-          :root[sizemode="normal"][tabsintitlebar] #titlebar,
-          :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+          :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+          :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
             appearance: none !important;
           }
           .browser-toolbar:not(.titlebar-color) {
@@ -6015,14 +6026,14 @@
             (-moz-os-version: windows-win8),
             (-moz-os-version: windows-win10),
             (-moz-platform: windows) {
-            :root[sizemode="maximized"][tabsintitlebar] {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
               --uc-titlebar-padding: 8px;
             }
-            :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
               margin-top: calc(-1 * var(--uc-titlebar-padding));
             }
-            :root[sizemode="normal"][tabsintitlebar] #titlebar,
-            :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+            :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
               appearance: none !important;
             }
             .browser-toolbar:not(.titlebar-color) {
@@ -6182,14 +6193,14 @@
             (-moz-os-version: windows-win8),
             (-moz-os-version: windows-win10),
             (-moz-platform: windows) {
-            :root[sizemode="maximized"][tabsintitlebar] {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
               --uc-titlebar-padding: 8px;
             }
-            :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
               margin-top: calc(-1 * var(--uc-titlebar-padding));
             }
-            :root[sizemode="normal"][tabsintitlebar] #titlebar,
-            :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+            :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
               appearance: none !important;
             }
             .browser-toolbar:not(.titlebar-color) {
@@ -6359,14 +6370,14 @@
             screen and (max-width: 1100px) and (-moz-os-version: windows-win8),
             screen and (max-width: 1100px) and (-moz-os-version: windows-win10),
             screen and (max-width: 1100px) and (-moz-platform: windows) {
-            :root[sizemode="maximized"][tabsintitlebar] {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
               --uc-titlebar-padding: 8px;
             }
-            :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
               margin-top: calc(-1 * var(--uc-titlebar-padding));
             }
-            :root[sizemode="normal"][tabsintitlebar] #titlebar,
-            :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+            :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+            :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
               appearance: none !important;
             }
             .browser-toolbar:not(.titlebar-color) {
@@ -6543,14 +6554,14 @@
               screen and (min-width: 1100px) and (-moz-os-version: windows-win8),
               screen and (min-width: 1100px) and (-moz-os-version: windows-win10),
               screen and (min-width: 1100px) and (-moz-platform: windows) {
-              :root[sizemode="maximized"][tabsintitlebar] {
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
                 --uc-titlebar-padding: 8px;
               }
-              :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
                 margin-top: calc(-1 * var(--uc-titlebar-padding));
               }
-              :root[sizemode="normal"][tabsintitlebar] #titlebar,
-              :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+              :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
                 appearance: none !important;
               }
               .browser-toolbar:not(.titlebar-color) {
@@ -6713,14 +6724,14 @@
               screen and (min-width: 1100px) and (-moz-os-version: windows-win8),
               screen and (min-width: 1100px) and (-moz-os-version: windows-win10),
               screen and (min-width: 1100px) and (-moz-platform: windows) {
-              :root[sizemode="maximized"][tabsintitlebar] {
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
                 --uc-titlebar-padding: 8px;
               }
-              :root[sizemode="maximized"][tabsintitlebar] #navigator-toolbox-background {
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #navigator-toolbox-background {
                 margin-top: calc(-1 * var(--uc-titlebar-padding));
               }
-              :root[sizemode="normal"][tabsintitlebar] #titlebar,
-              :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+              :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+              :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
                 appearance: none !important;
               }
               .browser-toolbar:not(.titlebar-color) {
@@ -7144,7 +7155,7 @@
 }
 /*= Tab Bar - Show only current tab ==========================================*/
 @supports -moz-bool-pref("userChrome.tabbar.as_titlebar") {
-  :root:not([tabsintitlebar="true"]) #tabbrowser-tabs,
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"]) #tabbrowser-tabs,
   #scrollbutton-up,
   #scrollbutton-down,
   .titlebar-spacer[type="pre-tabs"],
@@ -7213,8 +7224,10 @@
     }
   }
   @supports -moz-bool-pref("browser.tabs.tabmanager.enabled") {
-    :root:not([tabsintitlebar="true"]) #tabbrowser-tabs:not([overflow="true"], [hashiddentabs]) ~ #alltabs-button,
-    :root:not([tabsintitlebar="true"])
+    :root:not([tabsintitlebar="true"]):not([customtitlebar="true"])
+      #tabbrowser-tabs:not([overflow="true"], [hashiddentabs])
+      ~ #alltabs-button,
+    :root:not([tabsintitlebar="true"]):not([customtitlebar="true"])
       #tabbrowser-tabs[hasadjacentnewtabbutton]:not([overflow="true"])
       ~ #new-tab-button {
       display: flex !important;
@@ -8468,13 +8481,16 @@
     --uc-newtab-bgcolor: var(--toolbarbutton-active-background);
   }
   @media (-moz-windows-accent-color-in-titlebar) {
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme)) #tabs-newtab-button:hover,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      #tabs-newtab-button:hover,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      #tabs-newtab-button:hover {
       fill: color-mix(in srgb, AccentColorText 10%, transparent) !important; /* Hardcoded for compatibility */
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
       #tabs-newtab-button:hover:active,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      #tabs-newtab-button:hover:active {
       fill: color-mix(in srgb, AccentColorText 15%, transparent) !important; /* Hardcoded for compatibility */
     }
     @supports -moz-bool-pref("userChrome.tab.bottom_rounded_corner") {
@@ -8483,9 +8499,10 @@
         ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.chrome_legacy") or -moz-bool-pref(
           "userChrome.tab.bottom_rounded_corner.chrome"
         ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.edge") {
-        :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
           #tabs-newtab-button:hover,
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+          #tabs-newtab-button:hover {
           background-image: var(--uc-tab-corner-left-side-svg),
             linear-gradient(
               to left,
@@ -8494,9 +8511,9 @@
             ),
             var(--uc-tab-corner-right-side-svg);
         }
-        :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
           #tabs-newtab-button:hover:active,
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
           #tabs-newtab-button:hover:active {
           background-image: var(--uc-tab-corner-left-side-svg),
             linear-gradient(
@@ -8509,18 +8526,19 @@
       }
     }
     @supports -moz-bool-pref("userChrome.compatibility.accent_color") {
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
         #tabs-newtab-button:hover,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+        #tabs-newtab-button:hover {
         fill: color-mix(
           in srgb,
           -moz-accent-color-foreground 10%,
           transparent
         ) !important; /* Hardcoded for compatibility */
       }
-      :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
         #tabs-newtab-button:hover:active,
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
         #tabs-newtab-button:hover:active {
         fill: color-mix(
           in srgb,
@@ -8534,9 +8552,13 @@
           ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.chrome_legacy") or -moz-bool-pref(
             "userChrome.tab.bottom_rounded_corner.chrome"
           ) or -moz-bool-pref("userChrome.tab.bottom_rounded_corner.edge") {
-          :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+          :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(
+              :not([lwtheme]),
+              :not(:-moz-lwtheme)
+            )
             #tabs-newtab-button:hover,
-          :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+          :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+            #tabs-newtab-button:hover {
             background-image: var(--uc-tab-corner-left-side-svg),
               linear-gradient(
                 to left,
@@ -8545,9 +8567,12 @@
               ),
               var(--uc-tab-corner-right-side-svg);
           }
-          :root[tabsintitlebar]:not(:-moz-window-inactive):is(:not([lwtheme]), :not(:-moz-lwtheme))
+          :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive):is(
+              :not([lwtheme]),
+              :not(:-moz-lwtheme)
+            )
             #tabs-newtab-button:hover:active,
-          :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
+          :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)
             #tabs-newtab-button:hover:active {
             background-image: var(--uc-tab-corner-left-side-svg),
               linear-gradient(
@@ -10400,7 +10425,7 @@
     }
     @media (-moz-gtk-csd-available) {
       /* Fix transparent background */
-      :root[tabsintitlebar][sizemode="fullscreen"]:not([inDOMFullscreen="true"]):is(
+      :root:is([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"]:not([inDOMFullscreen="true"]):is(
           :not([lwtheme]),
           :not(:-moz-lwtheme)
         )
@@ -10654,16 +10679,16 @@
     }
   }
   @supports selector(:has(a)) {
-    :root:not([tabsintitlebar]):has(#toolbar-menubar) {
+    :root:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar) {
       --uc-menubar-height: var(--uc-menubar-height-default);
     }
     @media (-moz-gtk-csd-available) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) + 2 * var(--tab-block-margin));
       }
     }
     @media (-moz-os-version: windows-win10), (-moz-platform: windows-win10) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) - var(--tabs-navbar-shadow-size));
       }
     }
@@ -10671,15 +10696,15 @@
       (-moz-platform: windows-win7),
       (-moz-os-version: windows-win8),
       (-moz-platform: windows-win8) {
-      :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         --uc-menubar-height: calc(var(--tab-min-height) + var(--tab-block-margin));
       }
     }
-    :root[tabsintitlebar]:has(#toolbar-menubar[autohide="false"]) {
+    :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="false"]) {
       --uc-menubar-height: var(--uc-titlebar-buttonbox-height);
     }
-    :root:not([tabsintitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]),
-    :root[tabsintitlebar]:has(#toolbar-menubar[autohide="true"][inactive="true"]) {
+    :root:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]),
+    :root:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"][inactive="true"]) {
       --uc-menubar-height: 0px;
     }
     #navigator-toolbox:has(#PersonalToolbar[collapsed="true"]) {
diff --git a/src/autohide/_common.scss b/src/autohide/_common.scss
index 35ed7d7..7909372 100644
--- a/src/autohide/_common.scss
+++ b/src/autohide/_common.scss
@@ -48,11 +48,11 @@
   }
   @include Has {
     :root {
-      &:not([tabsintitlebar]):has(#toolbar-menubar) {
+      &:not([tabsintitlebar]):not([customtitlebar]):has(#toolbar-menubar) {
         --uc-menubar-height: var(--uc-menubar-height-default); // text 1rem, menu padding 0.3em
       }
 
-      &[tabsintitlebar]:has(#toolbar-menubar[autohide="true"]) {
+      &:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="true"]) {
         @include OS($linux) {
           --uc-menubar-height: calc(var(--tab-min-height) + 2 * var(--tab-block-margin));
         }
@@ -63,12 +63,12 @@
           --uc-menubar-height: calc(var(--tab-min-height) + var(--tab-block-margin));
         }
       }
-      &[tabsintitlebar]:has(#toolbar-menubar[autohide="false"]) {
+      &:is([tabsintitlebar], [customtitlebar]):has(#toolbar-menubar[autohide="false"]) {
         --uc-menubar-height: var(--uc-titlebar-buttonbox-height);
       }
 
-      &:not([tabsintitlebar]),
-      &[tabsintitlebar] {
+      &:not([tabsintitlebar]):not([customtitlebar]),
+      &:is([tabsintitlebar], [customtitlebar]) {
         &:has(#toolbar-menubar[autohide="true"][inactive="true"]) {
           --uc-menubar-height: 0px;
         }
diff --git a/src/compatibility/_os.scss b/src/compatibility/_os.scss
index db4ace7..67f03f9 100644
--- a/src/compatibility/_os.scss
+++ b/src/compatibility/_os.scss
@@ -1,6 +1,6 @@
 /*= Windows 10 - Top border of accent color at ESR #358 ======================*/
 @include OS($win10) {
-  :root[sizemode=normal][tabsintitlebar] {
+  :root[sizemode=normal]:is([tabsintitlebar], [customtitlebar]) {
     #navigator-toolbox {
       border-top: .5px solid rgb(47, 47, 47) !important;
     }
@@ -231,7 +231,7 @@ $_os_linuxDefaultShadow: 0 0 4px rgba(128, 128, 142, 0.5);
 
 /*= Titlebar Container Size at maximized #384 ================================*/
 @include OS($win) {
-  :root[tabsintitlebar] {
+  :root:is([tabsintitlebar], [customtitlebar]) {
     @include NotOption("userChrome.compatibility.os.windows_maximized") {
       #titlebar {
         // Prevent #476 FF v105 above
@@ -263,21 +263,21 @@ $_os_linuxDefaultShadow: 0 0 4px rgba(128, 128, 142, 0.5);
 // https://github.com/mozilla/gecko-dev/blob/4334cb91a3762ac497c50a1c7b86b8004690229e/browser/themes/windows/browser-aero.css#L71-L81
 @include OS($win) {
   @include WinClassic {
-    :root[tabsintitlebar][sizemode="normal"]:not(:-moz-lwtheme) #navigator-toolbox {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"]:not(:-moz-lwtheme) #navigator-toolbox {
       appearance: auto !important;
       -moz-default-appearance: -moz-window-titlebar !important;;
     }
 
-    :root[tabsintitlebar][sizemode="maximized"]:not(:-moz-lwtheme) #navigator-toolbox {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"]:not(:-moz-lwtheme) #navigator-toolbox {
       appearance: auto !important;;
       -moz-default-appearance:-moz-window-titlebar !important;;
     }
 
-    :root[tabsintitlebar]:not(:-moz-lwtheme) #tabbrowser-arrowscrollbox {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-lwtheme) #tabbrowser-arrowscrollbox {
       color: -moz-dialogtext !important;
     }
 
-    :root[tabsintitlebar][sizemode="normal"] #navigator-toolbox:-moz-lwtheme {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #navigator-toolbox:-moz-lwtheme {
       background-image: linear-gradient(
         to bottom,
         ThreeDLightShadow 0, ThreeDLightShadow 1px,
diff --git a/src/compatibility/_theme.scss b/src/compatibility/_theme.scss
index 6daa9dc..a987fe4 100644
--- a/src/compatibility/_theme.scss
+++ b/src/compatibility/_theme.scss
@@ -35,7 +35,7 @@ menu.subviewbutton {
 }
 
 /*= First visible tab margin at maximized #332 ===============================*/
-:root[tabsintitlebar="true"][sizemode="maximized"] #TabsToolbar {
+:root:is([tabsintitlebar="true"], [customtitlebar="true"])[sizemode="maximized"] #TabsToolbar {
   margin-left: -1px;
 }
 
@@ -193,9 +193,9 @@ vbox[part="drop-indicator-bar"] > image[part="drop-indicator"] {
 }
 
 /*= Inactive transparent #882 ================================================*/
-:root[tabsintitlebar] #titlebar:-moz-window-inactive,
-:root[tabsintitlebar] #toolbar-menubar:-moz-window-inactive,
-:root[tabsintitlebar] #TabsToolbar:-moz-window-inactive {
+:root:is([tabsintitlebar], [customtitlebar]) #titlebar:-moz-window-inactive,
+:root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar:-moz-window-inactive,
+:root:is([tabsintitlebar], [customtitlebar]) #TabsToolbar:-moz-window-inactive {
   opacity: 1 !important;
   color: color-mix(in srgb, currentColor calc(var(--inactive-titlebar-opacity, 0.75) * 100%), transparent) !important;
   --inactive-toolbarbutton-icon-fill-opacity: calc(var(--toolbarbutton-icon-fill-opacity, 1) * var(--inactive-titlebar-opacity, 0.75));
diff --git a/src/fullscreen/_overlap.scss b/src/fullscreen/_overlap.scss
index dc1d49c..0f5669c 100644
--- a/src/fullscreen/_overlap.scss
+++ b/src/fullscreen/_overlap.scss
@@ -18,7 +18,7 @@
 
 @include OS($linux) {
   /* Fix transparent background */
-  :root[tabsintitlebar][sizemode="fullscreen"]:not([inDOMFullscreen="true"])#{$not_lwtheme} #TabsToolbar {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="fullscreen"]:not([inDOMFullscreen="true"])#{$not_lwtheme} #TabsToolbar {
     appearance: auto !important;
   }
 }
diff --git a/src/padding/_drag_space.scss b/src/padding/_drag_space.scss
index 07e0f75..fd3c427 100644
--- a/src/padding/_drag_space.scss
+++ b/src/padding/_drag_space.scss
@@ -6,11 +6,11 @@
 
 //-- Mixin ---------------------------------------------------------------------
 @mixin _dragSpaceHeight {
-  :root[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"] {
+  :root:is([tabsintitlebar], [customtitlebar])[sizemode="normal"] #toolbar-menubar[autohide="true"] {
     @content;
   }
   @include Option("userChrome.padding.drag_space.maximized") {
-    :root[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] {
+    :root:is([tabsintitlebar], [customtitlebar])[sizemode="maximized"] #toolbar-menubar[autohide="true"] {
       @content;
     }
   }
diff --git a/src/padding/_first_tab.scss b/src/padding/_first_tab.scss
index 508a964..5684296 100644
--- a/src/padding/_first_tab.scss
+++ b/src/padding/_first_tab.scss
@@ -4,14 +4,14 @@
 }
 
 @include NotOption("userChrome.padding.first_tab.always") {
-  :root:not([tabsintitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
+  :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs:not([positionpinnedtabs]),
   :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs:not([positionpinnedtabs]) {
     padding-inline-start: var(--uc-space-left-tabbar) !important;
   }
 }
 
 @include Option("userChrome.padding.first_tab.always") {
-  :root:not([tabsintitlebar]) #tabbrowser-tabs,
+  :root:not([tabsintitlebar]):not([customtitlebar]) #tabbrowser-tabs,
   :root:is([sizemode="maximized"], [sizemode="fullscreen"]) #tabbrowser-tabs {
     padding-inline-start: calc(var(--tab-overflow-pinned-tabs-width, 0px) +  var(--uc-space-left-tabbar)) !important;
   }
diff --git a/src/padding/_tabbar_height.scss b/src/padding/_tabbar_height.scss
index 8b0b6a9..1ebf47b 100644
--- a/src/padding/_tabbar_height.scss
+++ b/src/padding/_tabbar_height.scss
@@ -94,7 +94,7 @@
   border-radius: var(--scrollbtn-border-radius, calc(var(--tab-border-radius) + 4px)) !important;
 }
 
-:root[tabsintitlebar]:not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
+:root:is([tabsintitlebar], [customtitlebar]):not([uidensity="compact"]) #toolbar-menubar[autohide="true"] {
   height: calc(
     var(--tab-min-height) - var(--tabs-navbar-shadow-size) - 2px
   ); /* Compact: 28px, Normal: 33px, Touch: 38px */
diff --git a/src/tab/newtab_button/_looks_like_tab.scss b/src/tab/newtab_button/_looks_like_tab.scss
index f6e7c8c..5450971 100644
--- a/src/tab/newtab_button/_looks_like_tab.scss
+++ b/src/tab/newtab_button/_looks_like_tab.scss
@@ -73,16 +73,16 @@
 }
 @media (-moz-windows-accent-color-in-titlebar) {
   @include AccentColor {
-    :root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
       fill: color-mix(
         in srgb,
         $accentTextColor 10%,
         transparent
       ) !important; /* Hardcoded for compatibility */
     }
-    :root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover:active,
-    :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+    :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover:active,
+    :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
       fill: color-mix(
         in srgb,
         $accentTextColor 15%,
@@ -98,14 +98,14 @@
         "userChrome.tab.bottom_rounded_corner.chrome",
         "userChrome.tab.bottom_rounded_corner.edge"
       ) {
-        :root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover,
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover,
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover {
           background-image: var(--uc-tab-corner-left-side-svg),
                             linear-gradient(to left, color-mix(in srgb, $accentTextColor 10%, transparent), color-mix(in srgb, $accentTextColor 10%, transparent)),
                             var(--uc-tab-corner-right-side-svg);
         }
-        :root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover:active,
-        :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
+        :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme} #tabs-newtab-button:hover:active,
+        :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) #tabs-newtab-button:hover:active {
           background-image: var(--uc-tab-corner-left-side-svg),
                             linear-gradient(to left, color-mix(in srgb, $accentTextColor 15%, transparent), color-mix(in srgb, $accentTextColor 15%, transparent)),
                             var(--uc-tab-corner-right-side-svg);
diff --git a/src/tabbar/_as_titlebar.scss b/src/tabbar/_as_titlebar.scss
index bd0c6cd..01aa0fc 100644
--- a/src/tabbar/_as_titlebar.scss
+++ b/src/tabbar/_as_titlebar.scss
@@ -12,7 +12,7 @@
 
 //------------------------------------------------------------------------------
 
-:root:not([tabsintitlebar="true"]) #tabbrowser-tabs, /* Show only */
+:root:not([tabsintitlebar="true"]):not([customtitlebar="true"]) #tabbrowser-tabs, /* Show only */
 #scrollbutton-up, /* Defaults */
 #scrollbutton-down,
 .titlebar-spacer[type="pre-tabs"],
@@ -80,8 +80,8 @@ spacer:is([part="overflow-start-indicator"], [part="overflow-end-indicator"]),
 
 
 @include Option("browser.tabs.tabmanager.enabled") {
-  :root:not([tabsintitlebar="true"]) #tabbrowser-tabs:not([overflow="true"], [hashiddentabs]) ~ #alltabs-button,
-  :root:not([tabsintitlebar="true"]) #tabbrowser-tabs[hasadjacentnewtabbutton]:not([overflow="true"]) ~ #new-tab-button {
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"]) #tabbrowser-tabs:not([overflow="true"], [hashiddentabs]) ~ #alltabs-button,
+  :root:not([tabsintitlebar="true"]):not([customtitlebar="true"]) #tabbrowser-tabs[hasadjacentnewtabbutton]:not([overflow="true"]) ~ #new-tab-button {
     @include Box(true);
   }
 }
diff --git a/src/tabbar/_on_bottom.scss b/src/tabbar/_on_bottom.scss
index 55080d9..456a535 100644
--- a/src/tabbar/_on_bottom.scss
+++ b/src/tabbar/_on_bottom.scss
@@ -82,7 +82,7 @@ See the above repository for updates as well as full license text. */
     width: 100%;
     overflow: hidden;
   }
-  :root[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
+  :root:is([tabsintitlebar], [customtitlebar]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) {
     height: var(--uc-menubar-height) !important; /* calc(var(--tab-min-height) + 2 * var(--tab-block-margin)) */
   }
 
diff --git a/src/tabbar/layout/_window_control.scss b/src/tabbar/layout/_window_control.scss
index f86f624..6408745 100644
--- a/src/tabbar/layout/_window_control.scss
+++ b/src/tabbar/layout/_window_control.scss
@@ -53,7 +53,7 @@ See the above repository for updates as well as full license text. */
 }
 
 @include OS($win) {
-  :root[sizemode="maximized"][tabsintitlebar] {
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) {
     @if mode.isESR() {
       --uc-titlebar-padding: 8px;
     }
@@ -62,8 +62,8 @@ See the above repository for updates as well as full license text. */
       margin-top: calc(-1 * var(--uc-titlebar-padding));
     }
   }
-  :root[sizemode="normal"][tabsintitlebar] #titlebar,
-  :root[sizemode="maximized"][tabsintitlebar] #titlebar {
+  :root[sizemode="normal"]:is([tabsintitlebar], [customtitlebar]) #titlebar,
+  :root[sizemode="maximized"]:is([tabsintitlebar], [customtitlebar]) #titlebar {
     appearance: none !important;
   }
   .browser-toolbar:not(.titlebar-color) {
diff --git a/src/tabbar/layout/_window_control_size.scss b/src/tabbar/layout/_window_control_size.scss
index 6407d1b..d9c9a62 100644
--- a/src/tabbar/layout/_window_control_size.scss
+++ b/src/tabbar/layout/_window_control_size.scss
@@ -40,7 +40,7 @@
 
 //------------------------------------------------------------------------------
 
-:root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+:root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
   --uc-window-drag-space-pre: var(--uc-title-pre-spacer, 40px); // 30px
   --uc-window-drag-space-post: var(--uc-title-post-spacer, 40px); // 25px
 
@@ -71,7 +71,7 @@
   }
 }
 @include OS($mac) {
-  :root[tabsintitlebar] {
+  :root:is([tabsintitlebar], [customtitlebar]) {
     --uc-window-control-width: 72px;
   }
   :root[sizemode="fullscreen"] {
@@ -79,12 +79,12 @@
   }
 }
 
-:root:is([tabsintitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
+:root:is([tabsintitlebar][sizemode="maximized"], [customtitlebar][sizemode="maximized"], [sizemode="fullscreen"]) {
   @include _remove_spacer_pre;
 }
 
 @include Option("userChrome.tabbar.one_liner") {
-  :root:is([tabsintitlebar], [sizemode="fullscreen"]) {
+  :root:is([tabsintitlebar], [customtitlebar], [sizemode="fullscreen"]) {
     @include NotOption("userChrome.tabbar.one_liner.responsive") {
       @include _remove_spacer_oneLiner;
     }
diff --git a/src/theme/system_default_theme/_urlbar_focus_color.scss b/src/theme/system_default_theme/_urlbar_focus_color.scss
index 029b598..eefdd96 100644
--- a/src/theme/system_default_theme/_urlbar_focus_color.scss
+++ b/src/theme/system_default_theme/_urlbar_focus_color.scss
@@ -1,7 +1,7 @@
 /*= Common - URL Bar focus color =============================================*/
 // TODO: `OS` Not covered case
-$_urlBarFocusSelector: ":root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme}";
-$_urlBarFocusDarkSelector: ":root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)";
+$_urlBarFocusSelector: ":root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme}";
+$_urlBarFocusDarkSelector: ":root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive)";
 
 @media (-moz-windows-accent-color-in-titlebar), (-moz-gtk-csd-available) {
   /* URL Bar */
diff --git a/src/theme/system_default_theme/_win10.scss b/src/theme/system_default_theme/_win10.scss
index 48d53be..be2e1af 100644
--- a/src/theme/system_default_theme/_win10.scss
+++ b/src/theme/system_default_theme/_win10.scss
@@ -244,8 +244,8 @@
 
   @include Option("userChrome.theme.system_default") {
     @media (-moz-windows-accent-color-in-titlebar) {
-      :root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme},
-      :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+      :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme},
+      :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
         --toolbarseparator-color: color-mix(in srgb, currentColor 20%, transparent) !important; /* As default */
       }
     }
@@ -371,8 +371,8 @@
 /*= Windows10 - Titlebar accent color ========================================*/
 @media (-moz-windows-accent-color-in-titlebar) {
   /* Tab Bar */
-  :root[tabsintitlebar]:not(:-moz-window-inactive)#{$not_lwtheme},
-  :root[tabsintitlebar][lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
+  :root:is([tabsintitlebar], [customtitlebar]):not(:-moz-window-inactive)#{$not_lwtheme},
+  :root:is([tabsintitlebar], [customtitlebar])[lwt-default-theme-in-dark-mode]:not(:-moz-window-inactive) {
     .titlebar-color {
       @include AccentColor {
         color: $accentTextColor;