Port fixes from the trunk.

browser/base/content/browser.js

@@ -4348,6 +4348,9 @@ function makeURLAbsolute( base, url )
 
 function isContentFrame(aFocusedWindow)
 {
+  if (!aFocusedWindow)
+    return false;
+
   var focusedTop = Components.lookupMethod(aFocusedWindow, 'top')
                              .call(aFocusedWindow);