What are the other competing browsers? There's chrome(and the derivatives), safari, firefox? safari exists only because of ios lockin. Aren't most other browsers an increasingly smaller share? Genuine question.
Firefox is a little slow for an internal application we have that loads 40,000 rows of data into a grid but otherwise all our stuff works with it because I develop Firefox first and I think a few of us are all that really stands between Firefox and oblivion and probably are doing more work than a lot of the people they have on the payroll.
If you have specific sites that aren't working, please let us know and we can investigate and try to fix them.
The usual reporting channels are using https://webcompat.com or the "Report Broken Site" tool in the Firefox menu. Of course I"m also happy to take bug reports here if you (or anyone else) have them.