Add: Vertical Tab - userChrome.tab.as_titlebar

src/tabbar/_as_titlebar.scss

@@ -0,0 +1,43 @@
+:root:not([tabsintitlebar="true"]) #tabbrowser-tabs, /* Show only */
+#scrollbutton-up, /* Defaults */
+#scrollbutton-down,
+.titlebar-spacer[type="pre-tabs"],
+spacer[part="overflow-end-indicator"],
+.tabbrowser-tab:not([selected="true"]),
+.tabbrowser-tab[selected="true"] .tab-background,
+.tabbrowser-tab[selected="true"] .tab-stack:is(::before, ::after),
+.tabbrowser-tab[selected="true"] .tab-close-button,
+#tabs-newtab-button {
+  display: none !important;
+}
+
+.tabbrowser-tab[selected="true"] {
+  -moz-window-dragging: drag;
+  --tab-max-width: 100%;
+}
+.tabbrowser-tab[selected="true"] .tab-label-container {
+  margin-inline: 0 !important;
+}
+
+.tab-content {
+  margin-inline: auto;
+  width: 100%;
+}
+
+
+/* Pinned */
+#tabbrowser-tabs {
+  padding-inline: 0 !important; /* Original: var(--tab-overflow-pinned-tabs-width) 0; */
+}
+.tab-throbber,
+.tab-icon-pending,
+.tab-icon-image,
+.tab-sharing-icon-overlay,
+.tab-icon-overlay {
+  &[pinned] {
+    margin-inline-end: 5.5px;
+  }
+}
+.tab-label-container[pinned] {
+  width: unset !important;
+}

src/tabbar/_clipped_tab.scss

@@ -10,7 +10,9 @@
 
 /** Clipped tabs - Show close button at pinned tab ****************************/
 @include Option("userChrome.tab.close_button_at_pinned") {
-  @import "clipped_tab/pinned_close_button";
+  @include NotOption("userChrome.tab.as_titlebar") {
+    @import "clipped_tab/pinned_close_button";
+  }
 }
 
 /** Clipped tabs - Always show tab icon ***************************************/

src/tabbar/_index.scss

@@ -33,6 +33,11 @@
   @import "layout";
 }
 
+/*= Tab Bar - Show only current tab ==========================================*/
+@include Option("userChrome.tab.as_titlebar") {
+  @import "as_titlebar";
+}
+
 /*= Tab Bar - Connect to window ==============================================*/
 @include Option("userChrome.tab.connect_to_window") {
   @import "connect_to_window";