Firefox

Both of them are very well documented with sources and explanations in the comments. You could pick and choose what you need and understand to create your own. Speaking from experience, using these prefs without knowing what they do might cause you a headache down the road when something breaks.

Betterfox should be the one with least breakage.

Zen already makes use of a lot of Betterfox tweaks as I remember.

I don't believe you can use both as they both modify the same files, and would overwrite each other.

Arkenfox is geared more towards privacy at the expense of usability, so sites will break a lot more often. And there's no easy way to turn it off for only 1 site.

Personally after trying Arkenfox in the past, as well as Librewolf, it's just far too much effort dealing with the sheer amount of sites and features that stop working properly.