--- a/toolkit/content/customizeToolbar.js
+++ b/toolkit/content/customizeToolbar.js
@@ -138,17 +138,17 @@ function getToolbarAt(i)
 }
 
 /**
  * Persist the current set of buttons in all customizable toolbars to
  * localstore.
  */
 function persistCurrentSets()
 {
-  if (!gToolboxChanged)
+  if (!gToolboxChanged || gToolboxDocument.defaultView.closed)
     return;
 
   var customCount = 0;
   for (var i = 0; i < gToolbox.childNodes.length; ++i) {
     // Look for customizable toolbars that need to be persisted.
     var toolbar = getToolbarAt(i);
     if (isCustomizableToolbar(toolbar)) {
       // Calculate currentset and store it in the attribute.