If I use arkenfox and make a bunch of overrides, do I need to make sure to back it up and manually install on every fresh installation or does Firefox Sync take care of that?
Nope it doesn’t
Manually
What about extension settings? Is there a universal rule or is it per extension and up to them?
There is a ability, iirc for extensions to use your Mozilla Sync account for storing settings. However it´s up to each extension developr to include this.
Yes, turns out you need to manually enable in ublock and then manually export your rules yourself. Better than nothing, I guess.
You have to take care of your backup. Mozilla takes care about ai and translations.
To be honest I think it’s better they don’t sync these kind of things that are “hacky”. Someone who breaks his Firefox and don’t remember how should be able to make a fresh install to fix everything.