Mozilla is also making 10x as much revenue as 15 years ago, has massively increased cash reserves, and is finally starting to diversify its income streams.
And it's absurd to put the fault for the declining market share purely at the feet of Mozillas leadership. Not to say they were blameless, but when Google threw its weight behind Chrome, including massive ad campaigns on billboards, pushing it on its web properties, and regularly breaking Firefox on their own websites for no technical reasons, you can't reasonably expect Firefox to just magically maintain their market share. That said, at the upper end you could look at Germany, where Firefox is still between 10-20% (depending on the estimate you look at), as what could have been achievable more broadly.