I've been suffering with this problem for a while as well. I have four extensions installed from Debian packages: HTTPS Everywhere, NoScript, Privacy Badger, and uBlock Origin. They all keep disappearing from the toolbar and stop functioning. I agree that this is an important bug, since besides being annoying, it can cause real privacy breaches.
It's not due to cruft in old profiles; I've deleted and regenerated new profiles. It hits me on two different Debian unstable systems. I'm currently seeing the problem with Firefox 83.0 from Debian package 83.0-1. Celejar
