The latest update (4.0.2) broke the extension functions. Here are instructions for rolling back to the previous version (3.5.23).
1. Uninstall X-notifier (remove) from your extensions.
2. Search for X-notifier in the firefox addons.
3. On the X-notifier page, do NOT click 'add to Firefox'. Instead scroll down and click 'Version Information'.
4. Click 'see complete version history'.
5. Mouse-over Version 3.5.23 and click 'add to Firefox'.
6. Restart Firefox and the old functionality is restored.

However, since Firefox automatically installs add-on updates, you will need to disable this so that X-notifier does not simply install 4.0.2 again. Here are the instructions:
1. Go to about:config in address bar. Accept the risk.
2. Search for extensions.update.autoUpdateDefault and toggle it to be disabled.

Done! Hope this helps.