Sure FF did screw up on its UI. Like so many applications nowadays. There once was a 'user interface guidelines' that brought us some standards regarding what menus and buttons to use, and where to place them. But apart from that the reason why browsers like Edge and Chrome are on the rise is simply because MS pushes Edge on every win10 computer, the user did not choose this browser or may even not use it. Chrome alike, try to install some third party software and high chance it installs Chrome on the side, if you don't uncheck the option.
And don't forget Google's behind Chrome. Edge is now based on Chrome, so Google can push its AMP - which by the way is a total rape of all coding standards and will eventually augment the time and thus price for yout IT department in maintainig these sites.