XN-forums Script serving X-notifier 3.x for Firefox
The XN-forums fix of X-notifier 4.2.4
You can download it here: XN-forums Script
(Left click and download using the button that will appear at top-right next to the Share button)
Just add it as external script. It will automatically replace the included script with the same name.
(Obviously Reloaded users can do the same, when the included script didn't reach the Reloaded update yet)