The rise and fall reflects user popularity. And Firefox is less popular than ever, with both the new generation who grew up only knowing Safari and Chrome, and also with the old generation who they pissed off by trying to copy the shitty parts of Chrome instead of staying true to what made them popular in the first place such as speed, light weight, and getting out of your way by not charging the UI every 6 weeks.
Sure, maybe everyone left Firefox because of the "what's new" tab and the UI changes. But maybe it's because Safari is default on some devices, and Chrome has the world biggest marketing company behind it.
Of course Google won the browser war mainly because they're World Heavyweight Advertising Moneybags World Champion, but Firefox also did itself no favors by annoying it's old and faithful userbase with often, unnecessary and unpopular UI changes a while ago.
