Mull has been officially discontinued by DivestOS back in December, along with most of their other projects. A fork by the name IronFox emerged but it's too new to know whether it's a real contender.
Just tried Zen Browser, it's interface maybe clean but user experience is very not zen. It was frustrating to find out how to remove all the Google and WhatsApp from he default layout. Left / right tab system is find but it needs a fly out/in button like IDEs have to terminal / console to compact the view.
Looks like LibreWolf is the one to transition to.
I'd recommend brave, pretty decent I would say
not if you like features of firefox like sync, containers, sideberry, manifest v2 (ublock origin), etc.