Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and check for that add-on. Also check with it's support site.

hookc said

Firefox v.39 (Mac 10.10.4) has mysteriously disabled my Alert for gMail which has been in use for a long time. Settings window says “browser disabled alerts“ which I didn’t do, and there’s no apparent way to restore the app.

Thanks, Fred ... problem not totally solved but now seem to be better focused on a solution.