I'm pretty sure you can sideload any extension, you might need to change something in the "hidden" advanced settings first though.

I just use Onyx so I've not done it myself yet but I will do when I can get onto Mutiny.

If you look up sideloading Firefox extensions on Android I'm sure there's info on how to do it. Firefox for Android is pretty much a full port of the desktop version so it can do it, just a matter of letting you install extensions from outside the official site.