It's not like FF was ever shipped with Windows. It's just that enough people saw enough value and purpose in it, to go out and get it themselves.

You can install FF on Android in a second on the Play Store, it's easier than it's ever been on the desktop; but you have to want to install it in the first place. Some of this want can be generated by feature excellence, some by peer pressure; but for a long time now, FF has lagged in producing the former and its leadership has actively sabotaged the community that can provide the latter.

Obviously FF was helped by MS dropping the ball in a way Google has not (yet), but in my opinion what happened then could be done again. Antitrust action is coming for both Apple and Google, Mozilla should be ready to pounce right there and then - but it cannot happen if the leadership does not understand that there is a problem.