The big problem is the browser engine at the heart of all browsers, all the FF or Chromium forks very rarely modify the core. When they do, it's minor stuff. That's why AFAIK not a single chromium fork is maintaining manifest v2 in defiance of Google.

If Mozilla goes full tilt enshittification, all the FF forks will suffer a similar fate, they'll make changes all over, custom interface, cool little features here and there etc; but they'll never make major changes to the core and that's assuming they keep the core open source. If they take the core closed source and the forks can no longer get upstream updates for it they'll wither and die

A browser engine is kinda like the Linux kernel, it's large, complex and takes a lot of time and effort to make and keep it usable. I've seen estimates that if we needed to start from scratch on the Linux kernel it'd take 2-4 years just to get something decently usable.

Browser engines are similar, Ladybird for example, is a new open source browser AND engine from scratch that's been in development for about 2 years, they're estimating to have something "generally usable" in 2026