There is a difference between forks made by other people to tweak a project/do something specific for it, and the base project's dev team moving away from whatever it became.

I'm usually not in favor of such fork because the reason for moving away is sometime dubious; some project just rename themselves to "start fresh and drop legacy compatibility issue". But in the case of Firefox, Mozilla is the thing holding back features while adding bloat. Since it can't change to a saner structure with more long-term sustainability plans, devs/engineers could move into a fork to not be bound to that anymore.

Of course it's not that easy; for all the bad Mozilla (foundation or other, I don't care much that they are two entities at this point since one is owned by the other) is doing to the actual software, they do provide salaries. At least, for now.