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.