Forking is an option only insofar as the forking team is actually capable of leading development. Purely reactive forks, like most of the ones we see in the Chromium world, are not sustainable in the long run. Is there a team, out there, who could fork FF and take it into a new direction? Maybe. But that has happened only once in the history of Mozilla, and it was paid for by Mozilla itself, because it's a huge task.