Yeah, I guess your point stands. But also, it’s 221 mio for Mozilla as a whole. Firefox might again be a fraction of this. While e.g. the Linux foundation has a lower budget, with all the contributed work hours of volunteers / corporations, a fork of Firefox is more realistic than the 500 mio make it out to be.
Well LoL has no official Linux support, so a low current number of users is no indication of the size of the potential Linux player base.