If you don't want to (or can't) update to the latest Firefox, then you can install the "Disabled Add-on Fix for Firefox" extension Mozilla just released.
Mozilla has a blog post about it, along with three versions of the same extension (the first three bullet points of the post.) Just be sure to choose the one which matches your version.
https://blog.mozilla.org/addons/2019...ns-in-firefox/
Cheers!