Bug 1077652 - tabbrowser should ignore DOMTitleChanged events from browser that have no tab assigned r=dao

browser/base/content/tabbrowser.xml

@@ -3465,7 +3465,7 @@
             return;
 
           var tab = this._getTabForContentWindow(contentWin);
-          if (tab.hasAttribute("pending"))
+          if (!tab || tab.hasAttribute("pending"))
             return;
 
           var titleChanged = this.setTabTitle(tab);