wall paper fix for bug #348183 nsGlobalWindow:Focus() can return NS_ERROR_FAILURE if we try to focus a disabled element. the unhandled exception in tabbrowser.xml can lead to problems, such as "canceling the 'do you want to close the window with open tabs' prompt still closes the window" r=bryner

browser/base/content/tabbrowser.xml

@@ -765,7 +765,12 @@
 
             function setFocus(element) {
               document.commandDispatcher.suppressFocusScroll = true;
-              element.focus();
+              try {
+                element.focus();
+              }
+              catch(ex) {
+                dump("XXX focus() failed, see bug #348183: ex = " + ex + "\n");
+              }
               document.commandDispatcher.suppressFocusScroll = false;
             }