diff --git a/src/fullscreen/_overlap.scss b/src/fullscreen/_overlap.scss
new file mode 100644
index 0000000..387c057
--- /dev/null
+++ b/src/fullscreen/_overlap.scss
@@ -0,0 +1,15 @@
+:root[sizemode="fullscreen"] #navigator-toolbox {
+  position: fixed !important; /* Needed for content to take up entire height */
+  z-index: 1000 !important; /* Puts the UI above the content */
+}
+
+:root[sizemode="fullscreen"] :-moz-any(#navigator-toolbox, #titlebar, #nav-bar, #PersonalToolbar) {
+  width: 100%; /* Makes the UI take up the entire width */
+}
+
+@include OS($linux) {
+  /* Fix transparent backgorund */
+  :root[tabsintitlebar][sizemode="fullscreen"] #TabsToolbar:not(:-moz-lwtheme) {
+    appearance: auto !important;
+  }
+}
diff --git a/src/fullscreen/_show_bookmarkbar.css b/src/fullscreen/_show_bookmarkbar.css
new file mode 100644
index 0000000..a91348e
--- /dev/null
+++ b/src/fullscreen/_show_bookmarkbar.css
@@ -0,0 +1,4 @@
+:root:not([sizemode="fullscreen"]) #PersonalToolbar[initialized="true"]:not([collapsed="true"]),
+:root[sizemode="fullscreen"] #PersonalToolbar[initialized="true"] {
+  visibility: unset !important; /* Makes the bookmarks toolbar visible if enabled */
+}