The solution for now appears to be to roll back the add-on to a previous version that worked for you. As has been posted before, the link to do that is:

https://addons.mozilla.org/en-US/firefox/addon/xnotifier/versions/

However, you also need to turn off the auto updates so it doesn't return to the version you don't want. People have been asking how to do that, and I wasn't sure myself, so here's the answer:

From the menu at the top of Firefox, go to Add-ons in the drop-down menu under Tools. That opens your Add-Ons Manager page.

Scroll to X-notifier and click on More.

Scroll to the bottom of that page and you should see Automatic Updates at the top of a list of options. Check the Off button.

That should keep it from updating until the issues are resolved.