Some manufacturers like Xiaomi already have a very annoying flow for enabling developer options and using ADB. You need to have a SIM card inserted, need to create a Xiaomi account and there's several popups with timers you have to wait through.
There’s also Waterfox and Librewolf (which are more vanilla).
There’s a problem, though: there’s little to no core development happening in any of these forks. If Mozilla comes crashing down, somebody will have to pick it up.
Why not an Android with termux? I guess it would depend upon your software stack, but my editor (Emacs) will have native Android port soon and I am stoked to try it. The latest Snapdragon processors are fairly powerful (and Mediatek/Dimensity ones aren't lagging behind either).