--- a/mail/base/content/msgMail3PaneWindow.js
+++ b/mail/base/content/msgMail3PaneWindow.js
@@ -1656,16 +1656,21 @@ var TabsInTitlebar = {
 
       this.allowedBy("sizemode", false);
       window.addEventListener("resize", function (event) {
         if (event.target != window)
           return;
         TabsInTitlebar.allowedBy("sizemode", true);
       }, false);
 
+      // Always disable on unsupported GTK versions.
+      if (AppConstants.MOZ_WIDGET_TOOLKIT == "gtk3") {
+        this.allowedBy("gtk", window.matchMedia("(-moz-gtk-csd-available)"));
+      }
+
       // We need to update the appearance of the titlebar when the menu changes
       // from the active to the inactive state. We can't, however, rely on
       // DOMMenuBarInactive, because the menu fires this event and then removes
       // the inactive attribute after an event-loop spin.
       //
       // Because updating the appearance involves sampling the heights and
       // margins of various elements, it's important that the layout be more or
       // less settled before updating the titlebar. So instead of listening to