If you want to stick to 3.5.23,

1. In Firefox's Add-ons Manager -> X-notifier -> More -> Set 'Automatic Update' to 'Off'

2. Install 3.5.23 from here https://addons.mozilla.org/en-US/firefox/addon/xnotifier/versions/

This version will stop working in the future according to Firefox team's plan.

IT WORKS :)