Do you have any good examples of sites that don’t work on Firefox? I hear this a lot, but I don’t seem to experience it. I exclusively use Firefox on the desktop, while I use Safari on mobile.
most of the time the sites "work". Issues are usually in one of two categories: (1) bad/ugly layout, (2) failure to login properly. Occasionally, web apps for smaller organizations will just stop me at the door due to my User Agent string.