Firefox's share was never very high to beginning with, because Mozilla doesn't have the resources to play the vendor bundled game that Microsoft and Google have been playing.
And now it's rising again, because they've made great improvements in Quantum and are on a roll. I use Firefox and it really is the best browser, at least for me. The only problem that Firefox has at this point is that its developer tools are worse than Chrome's, however they've been making improvements there as well and I'm sure they'll manage to win back the developers they've lost.
> Google's render can be found in Opera, Vivaldi, and others
You mean WebKit, which originated in KDE and then forked by Apple? It's not really Google's engine ;-)
Also Opera, Vivaldi and others are at this point completely irrelevant.
Mozilla is playing this game correctly. If you don't have your own rendering engine, you can't compete and you don't count. Which is also why Google developed V8 and then forked WebKit into Blink.