diff --git a/toolkit/content/widgets/toolbar.xml b/toolkit/content/widgets/toolbar.xml
index 67ec18929337..72e66c7b4358 100644
--- a/toolkit/content/widgets/toolbar.xml
+++ b/toolkit/content/widgets/toolbar.xml
@@ -430,15 +430,16 @@
   <binding id="toolbar-drag"
            extends="chrome://global/content/bindings/toolbar.xml#toolbar">
     <implementation>
-      <constructor>
+      <constructor><![CDATA[
         try {
           Components.utils.import("resource://gre/modules/WindowDraggingUtils.jsm");
           let draggableThis = new WindowDraggingElement(this, window);
           draggableThis.mouseDownCheck = function(e) {
-            return !this.parentNode.customizing; // Don't move while customizing.
+            // Don't move while customizing or while in full screen mode.
+            return !this.parentNode.customizing && !window.fullScreen;
           }
         } catch (e) {}
-      </constructor>
+      ]]></constructor>
     </implementation>
   </binding>