Because it is FOSS and responsible for many great contributions to apis that make the web what it is. It has history that goes way back. It has been decently transparent, certainly when compared to its closest competitors. It isn’t Google. It has a massive library of extensions. They aren’t planning to deprecate manifest v2.
Don’t get me wrong, I also like other browsers and I’m looking forward to seeing what comes from the servo reboot. But Firefox is bread and butter and there is often drummed up nonsense about it.
Stopped innovating? Just because the user interface didn’t change much? They’ve contributed a ton to web api’s and the open web in general. They also contributed massively to rust, and private / secure browsing standards. It has absolutely not been left to languish. Now I prefer some other UI’s but you won’t catch me claiming Mozilla ceased innovation.
They’ve also contributed in general to JavaScript. So yeah, Google definitely pushed the envelope there, but Mozilla didn’t just watch it all happen. Also, factor in that they were key contributors to web assembly.