> Firefox isn't doing great right now, but is it doing any worse than Brave?

Maybe in terms of market share as a proxy for success that is true.

As a browser though Firefox has come on leaps and bounds (and has momentum to keep improving), I never switched away from it but used Chrome for dev for the last few years so I have a fair amount of time with both (dozens of hours a week) and Firefox is as fast and reliable as Chrome.

Ironically the one property that gives me issues with Firefox is youtube, they don't fix bugs against Firefox (the white bar header on fullscreen has been in firefox for over a month now and has been reported).

I ended up having to implement a ublock origin rule to remove the header.

EDIT: Mozilla are (to me) so much more than just a browser vendor, MDN documentation is probably my 2nd most visited documentation/help site (behind only stack overflow) because the quality is superb, concise and they quickly mention at the bottom can do this on foo, bar but not fizz which is useful to someone approach 40 because keeping that stuff in my head is well beyond me these days.