all data of the firefox is located in /data/data/packgename, which is inaccessible without root permissions. a few android versions ago it was accessible through ADB, not anymore. I don’t think it’s possible anymore to install arkenfox on plain firefox android for most users.

even with a rooted phone its not that easy. granting termux root rights and running su, the per-package data folders are not visible, you have to switch to the correct mount namespace somehow. some file manager apps adapted to be able to do that