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&...