After a quick look at what I believe is mull's most up to date repo https://github.com/Divested-Mobile/Mull-Fenix
It doesn't appear to be too complex to maintain (only looked at the past 3 or so commits as I'm with fam right now)
Hoping someone continues with the project, as it was, in my opinion, probably the most privacy friendly browser for android other than Tor which has it's usability issues.
I may try to play around with setting up an fdroid build server for the interim after the holidays.
Edit: I wonder if the old maintainer would be open to having some sort of knowledge transfer session?