Not sure how to add a script from http://xnotifier.tobwithu.com/scripts.php , clicking the script name only allows to download the script and save it to the local disk.
You can only add scripts to Chrome/X-Notifier (any version) or to the old version 3.5.x X-Notifier in Firefox 56.0.2 or older. You cannot add scripts to Firefox with X-Notifier 4.x. After you download a script... To add a script, you can right-click on the X-Notifier 'X' icon, click Options, click the Scripts button, and click 'Add'... browse to where you saved the script, select it, click Open, and OK.
Adding scripts
You can only add scripts to Chrome/X-Notifier (any version) or to the old version 3.5.x X-Notifier in Firefox 56.0.2 or older. You cannot add scripts to Firefox with X-Notifier 4.x.
After you download a script...
To add a script, you can right-click on the X-Notifier 'X' icon, click Options, click the Scripts button, and click 'Add'... browse to where you saved the script, select it, click Open, and OK.