Same deal as Mullvad. Librewolf is just a fork of Firefox with the Arkenfox user.js already incorporated. It's super convenient, and it will get updates a bit slower than vanilla Firefox because the devs have to wait for Firefox to release their update and then patch Librewolf. It's a great option, though. I have a spare PC I use for certain tasks and I don't do much upkeep or browsing on it, so I use Librewolf there to avoid the hassle of remembering to update my user.js and all that junk. It does a great job balancing ease of use and privacy/security hardening, though I'll probably switch to Mullvad Browser at some point soon.
Iceraven is basically just vanilla Firefox. There's a couple small differences and it (probably) doesn't send telemetry back to Mozilla, but otherwise, you're not really getting much of a privacy benefit from using it and updates are slower than stock. The only beneficial thing is that the about:config section is available, but I think that's the case with Firefox Nightly, too (though I might be mistaken).
Mull is another fork that uses the arkenfox profile on mobile, which is really cool, but as of right now, Firefox doesn't offer proper site isolation on Android and isn't the most private option, unfortunately. As much as I dislike Brave for personal reasons, it's probably the best Android browser option today after a few settings tweaks. At least until Mozilla finally gets site isolation going on Android.