Not their fault of course, with people not testing websites on non chrome derived browsers.
Pretty bad as in that isn't true?
Firefox is the option that doesn't intentionally leave users vulnerable to hostile adtech. Firefox is the option with containers. Past that it is performant and reliable under a wide variety of user loads and platforms.
or Pretty bad as in Firefox+forks are better than the alternatives?
It is true that some unfortunate default options were recently added to Firefox configs.
Those options are unfortunate because they are variants of anti-user options baked into Chromium - options created to keep Chromium users susceptible to big-tech's worst intentions.
The difference between Firefox's 1x and Chromium's 100x + 100x is in the degree of harm visited upon the user.
Finding harsh fault with former while giving the much more egregious example a pass -- this makes sense if one feels Firefox isn't abusive enough towards it's users.