I don't think it's quite as simple as someone just forking it. Realistically, a browser is an extremely complex piece of software that requires a lot of organizational effort to maintain, deal with security issues, etc. Pretty much every other piece of software on a similar scale I can think of (the kernel, KDE, Blender, Libreoffice) has some sort of organization behind it with at least some amount of officially paid work. All the major forks of Firefox or chromium follow quite closely to upstream for this reason (which is also why I'm skeptical of Brave's ability to maintain manifest v2 long term, despite their probably genuine best efforts to do so).
I do wish that Firefox were developed and funded by an organization specifically dedicated to developing it. This could of course happen if Mozilla dies. But that's going to require someone starting it, which is not at all a small or cheap task.
I could also see a future where Oracle or IBM buys it 😂🤡
Winamp published their code as "open source". Problem is...