I checked, and according to Statcounter it's at 3.3%. So if Mozilla did go hardball, it'd affect an insignificant amount of people.

Realistically though, I don't follow world politics much but I assume that "blocking firefox" probably wouldn't be the worst optics they've had in the past few years.

If Mozilla gets blocked, people would just install some other browser (probably, something from Russia). I do not see how this helps anyone but the government itself. And departure of hundreds (if not thousands) of western companies did nothing to the Russian government, some problems with a browser with almost non-existent userbase would have the same effect. It should be quite clear by now that such tactic simply does not work.