Yep. I always mention Fennec who may hesitate to use Beta since it’s a stable release with full add-on support.
I’ve never tried Mull.
Not an expert but as I’ve understood it it’s pretty similar to Fennec, less tracking and more private than your regular Firefox. I don’t know anything about either ones development team size or activity either so I can’t recommend one over the other. Hopefully someone with more knowledge can chime in.
Mull’s made by the people at DivestOS
They have a browser comparison of some popular android browsers including Fennec and Mull https://divestos.org/pages/browsers