This bug [1] in recent versions of Firefox broke websockets for some sites (I don't know how many, but it included [2]). It was a major bug in their implementation of TLS 1.3 in Firefox, but its symptoms were subtle. You could connect the first time, but reconnecting would then be completely broken until you restarted Firefox (due to some sort of caching). For years, the Firefox flex css layout implementation was unbelievably slow [3], which basically broke usability of some sites. For the web application I work on, we've had to contend with over 10 Firefox-specific bugs [4]. I think everything works fine today though.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1453204

[2] https://cocalc.com

[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1209697

[4] https://github.com/sagemathinc/cocalc/issues?utf8=%E2%9C%93&...

Two that come to mind: Microsoft's "Seller dashboard" for managing Office 365 extensions, and Google's "Cloud Platform" console for managing their services. Also, Apple's "iTunesConnect" is pretty glitchy unless you use Safari (and probably Chrome too). I guess you can see a clear reason why each of those pages behaves like so :).

Edit to add that I prefer Firefox in general for philosophical and practical reasons: I'm used to the dev tools and have a hard time navigating the Chrome/Safari alternatives, not to mention Edge/Explorer.

The only site I had issues with is AWS DMS (Data Migration Service). I can't confirm it's still half-broken with Firefox as I don't use this service anymore.

But, to be fair, I also had the opposite issue with some other AWS services where it was working in Firefox but not on Chrome

I literally said don’t work, so I’m not sure why you’ve changed this to “literally do not load”. I also listed two that don’t work properly and I need to switch to chrome to use.

I use Firefox for almost every site and am I Firefox fan, but you can’t pretend it works flawlessly for ever one.

Seriously, besides the occasional webgl demo I haven't experienced this either.