Just my POV
I started with Opera and switch to FF way back when. About 3 years ago, it just got to the point where loading anything was a trial of patience, followed by wondering what else had been hosed during the latest update. Frankly, same reasons I stopped using Windows and Gnome at home: The changes made no sense and seemed to make things worse. I use Chrome since it "just works" for everything I need. Adding in ublock origins, blocksite and disable HTML autoplay and every site "just works" without bogging things down. I do give the odd version of FF a try to see if it's back up to par but it's still slow and loading my 'needed' extensions makes every page render a crawl. And that's on top of any other cruft. If FF and Thunderbird had just kept the focus on standards compliance, sleek internals and leaving the rest to plugins, I know I'd never have jumped ship.