There are various Firefox forks already (the ones I'm aware of are for old OSes) but there aren't enough people to sustain them.

When Firefox/phoenix was created, it was essentially by two people, Dave Hyatt and Blake Ross, isn't?

Firefox was based on the existing Mozilla codebase maintained by dozens or hundreds of engineers. Likewise, as long as Firefox is actively maintained by Mozilla it only takes 1-2 people to maintain a small fork. But if Mozilla goes out of business and Firefox becomes completely unmaintained all of the forks will also die.

Depending on your definition of "fork", Firefox has been forked in cases like the Tor Browser or IceWeasel. But starting a completely separate organization that could keep its isolated Firefox fork competitive with Chromium would require a massive amount of resources. Mozilla employs hundreds of full-time engineers (although that number is declining).

Yes, but the doe tho..

99% of the time, it's not a technical problem.

I'm running Waterfox, and I've heard good things about Basilisk for people who want/need to keep the old XUL ways alive.