--- a/mail/base/content/mailWindow.js
+++ b/mail/base/content/mailWindow.js
@@ -512,17 +512,17 @@ function getNotificationBox(aWindow) {
   var tabmail = document.getElementById("tabmail");
   var tabInfo = tabmail.tabInfo;
 
   for (var i = 0; i < tabInfo.length; ++i) {
     var browserFunc = tabInfo[i].mode.getBrowser ||
                       tabInfo[i].mode.tabType.getBrowser;
     if (browserFunc) {
       var possBrowser = browserFunc.call(tabInfo[i].mode.tabType, tabInfo[i]);
-      if (possBrowser && possBrowser.contentWindow == aWindow)
+      if (possBrowser && possBrowser.contentWindow == aWindow && possBrowser.parentNode.tagName == "notificationbox")
         return possBrowser.parentNode;
     }
   }
   return null;
 }
 
 // Given the server, open the twisty and the set the selection
 // on inbox of that server.