You need to activate the developers options/debug menu:
Settings -> About Fennec -> tap on the fennec logo multiple times
Go back to settings the option to install from files should appear.
Maybe I’m missing something but I don’t see any option to install from files with Fennec? Only from a collection. And it seems you can’t add bypass paywalls clean to a collection because it’s not in the AMO.
Edit: nvm had to update Fennec
Ah, and here I was thinking installing addons from file was impossible on Android. Thanks!