the most useful extensions for android firefox - ublock origin, there is already (and many more). Also, mozilla is working on bringing all desktop add-ons to mobile and everyone can contribute.
The Mull version of Firefox has always had extension support, never even knew the official version had even removed it. Mull is otherwise identical with some hardening against tracking and removed telemetry. I've also never had issues with sites breaking which can be otherwise common with hardening
pretty awesome, especially with extensions. I use it everyday.
In fact, I have two firefoxes. one of them reserved for slack as I don't want to download the app and slack detects that I'm using desktop mode on android(on chromium-based browsers). but Firefox's(with chameleon) spoofing saves my device from one of the worst proprietary applications.