if you install arkenfox yourself, likely you’ll never remember to update it. if you use librewolf or such, the maintainers will take care of it for you

I’m sure there are plugins that either check for updates for you, or remind you to do so (even though that can defeat the purpose some, of course). If not I’ll make one myself once I get around to setting arkenfox up. But yes, that’s largely what I meant by “While I understand that setting up a config is probably beyond the scope of your average browser user”.

and where do you copy it on your phone? I think it’s not that simple but maybe I missed something

Not sure, but considering you can mess around in about:config I’d assume the user.js is located in userspace somewhere. I haven’t looked into it much. If nothing else you can probably do it easily with a rooted phone.