For those who don’t know yet: Firefox Nightly for Android let you install any extension you desire.
You just need to create a custom collection on the web site and link your ID in nightly.
Like mine, for those who don’t want to curate: https://addons.mozilla.org/en-US/firefox/collections/17401236/p1xiekat-Android/
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.