Firefox has 2.82% marketshare as of May 2024. It'd create disruption to block it, but I'd expect that that's probably low enough that it's not in the "too big to kill" category.
If it were 2010, then yeah, I'd say that the price to pay for blocking Firefox is maybe one that's too high for the Kremlin to be willing to pay.
What's really clobbered Firefox has been the rise of smartphones, where Firefox has very limited uptake.
I use Firefox on both my phone and desktop, so I can say that it's definitely usable...but it's not the default. Google owns Android and uses their browser as the default, and Apple owns iOS and uses their browser as the default. I would bet that a very low proportion of smartphone users are ever going to seek out and install a different browser, and Firefox can only really compete for the users who are willing to do that.