However, I did do a lot of frontend back in the old IE6/IE7/IE8 days when you essentially had to code a whole separate front end for Microsoft's standards-flaunting mess. So this is definitely an issue I care about.

    but behind the curtains it took additional effort from 
    the developers of websites you visit to make it so

So yes, you often can't run your Chrome-specific shit elsewhere without workarounds and polyfills, but this doesn't automatically mean everybody except Google is screwing up. In some cases, complaints such as yours sound like folks in 2004 complaining that their ActiveX controls work in IE but not Firefox.

I'm pleased to see others have good experience with FF performance, but for me, the performance simply became unacceptable.

Even the fully local translation is really usable now <3. I don't use chrome for anything and it's not even installed on my daily driver anymore.

Ironically, the only time I have had issues, overriding the user agent to look like chrome or edge fixed it. So those websites were deliberately broken with Firefox, not Mozilla's fault but pure malice and dark patterns. Office 365 is one of these sites by the way.