Earlier quoted context omitted.
It's a catch-22 because if you stop innovating your UI for 20 years and alternatives come up with something people actually like then you will lose users to them and slowly fade into irrelevancy. Firefox succeeded because it was a fresh take on the entire browser UX at a time when Internet Explorer had been stagnant for half a decade.
As I remember it, Firefox succeeded because it fundamentally worked well and was very configurable, not because of the UX. The others at the time were bad at both of those things. The UX of Firefox was (and, I'd argue, still is) not great, but it made up for that by being configurable enough that you could fix it for yourself.
I would love for Firefox to be awesome, but ultimately (in my subjective opinion of course) it just kind of sucks and always has.