In my experience, Firefox actually has better performance than Chrome nowadays — particularly on Apple Silicon, where the difference is stark and dramatic. And with the new "Proton" interface, IMO Firefox finally looks at least as nice as Chrome out of the box.

Firefox mobile still has some catching up to do on the overall UI/UX, but it's not too far off, and the fact that it allows extensions is nice.

Safari and, especially, its iOS monopoly (for the engine, at least) are the only thing keeping Google from dominating and nearly everyone from only testing against Chrome, now that MS switched to using Chrome's engine and IE is finally nearing the end even in the important niches where it had been hanging on. FF doesn't have enough "normies" using it, isn't the default anywhere, and its trend-line is heading the wrong direction.

I'm not sure I'd classify it as viable with how Mozilla middle management has acted the last few years.