--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -666,16 +666,24 @@ notification[value="translation"] menuli
   /* Private browsing and accessibility indicators */
   :root[sizemode="normal"][chromehidden~="menubar"] #TabsToolbar > .private-browsing-indicator,
   :root[sizemode="normal"][chromehidden~="menubar"] #TabsToolbar > .accessibility-indicator,
   :root[sizemode="normal"] #toolbar-menubar[autohide="true"][inactive] + #TabsToolbar > .private-browsing-indicator,
   :root[sizemode="normal"] #toolbar-menubar[autohide="true"][inactive] + #TabsToolbar > .accessibility-indicator {
     padding-top: calc(-1 * var(--space-above-tabbar));
   }
 
+  /* Make #TabsToolbar transparent as we style underlying #titlebar with
+   * -moz-window-titlebar (Gtk+ theme).
+   */
+  :root[tabsintitlebar] #TabsToolbar,
+  :root[tabsintitlebar] #toolbar-menubar {
+    -moz-appearance: none;
+  }
+
   /* The button box must appear on top of the navigator-toolbox in order for
    * click and hover mouse events to work properly for the button in the restored
    * window state. Otherwise, elements in the navigator-toolbox, like the menubar,
    * can swallow those events.
    */
   #titlebar-buttonbox {
     z-index: 1;
     -moz-box-align: center;