The problem is that if Firefox (and derivatives)'s market share drop too low then developers won’t continue to support them. This is already happening to some extent with websites missing features or flat out refusing to run on Firefox.

Then you won’t be able to use Firefox to browse a lot of the web, and Google will basically have full control.

So Firefox being popular isn’t just good for Mozilla and the people that use it. It is critical for the open web in general. (Unless we can find another Chrome competitor)