Ever since Firefox began messing with their UI, following Chrome's development cycle, and nuked their extension ecosystem (which some things have yet to be reimplemented, resulting in some cross-browser add-ons having features that work in Chrome but NOT Firefox), they have just lost so much market share. I'm sure Google pushing Chrome hard and the skyrocketing of mobile didn't do Mozilla any favors either.
I've tried to go back to Firefox, but the scrolling does not adhere to native OS scrolling behavior, meaning you have to go into about:config and tweak it until you think it looks right.