It has included some privacy measures to resist fingerprinting like letterboxing and has more privacy focused search engines as default like searx. Also it takes out some firefox utilities like pocket which I don't really use

As for Mullwav browser I'm not really sure, it seems to be another reinforced firefox like librewolf

Btw, here is a detailed, technical review. It is in German, but with transtae and all the code, it should be understandable.

TLDR: It's good.

Firefox has a weaker sandbox than chromium and less mature site isolation and therefore has lower security. privacy is a different story, but remember you're only as private as you are secure so Firefox is inherently not that private assuming a malicious site escapes the sandbox.

I'm fully against chrome's growing monopoly as well as Google surveillance capitalism but let's not be so dramatic with the "google mother ship" nonsense.

using chromium as a base does not equal data being sent back to Google, just like using Android as a base doesn't inherently send data back to Google.

It is. Custom roms modify very little of the code and they are all based on aosp(it is open source but google controlls the changes). The whole point of aosp is to create the illusion of choice but if you really want to avoid using google spyware you have to give up on most apps or go to extreme lenghts to use an alternative. The grapheneos project is really cool and usefull but it only patches the inherent (intended)problems of android and doesnt provide a real solution.