In my case: several financial institutions that I have accounts with and lots of enterprise apps and gear with web UIs either break silently, have certain functions that don't work, or block with an error telling you to use a supported browser. (If they're nice, they even tell you which ones are supported.)

For a while, Slack didn't support all features on Firefox despite the technology being there, I don't know if this is still the case.

Shopify admin is one (but also one that's better to just not use if you have that option)

The version of Unit 4 that we use at work only works in Chrome.

That said, lately, that particular thing has been the only one that I need to use that outright doesn't work.

I think a number of Google technologies are slower in Firefox on purpose, but I just avoid them and keep telling competition authorities about their abuse of power.

Sooner or later they will get caught.

Godaddy complains with other browsers...

https://news.ycombinator.com/item?id=32093987

The Hebrew language Nespresso site pops up a browser dialogue "you have unsaved changes" every time one tries to browse to another page. The dev team is aware of it, but "the site only supports Google Chrome" so they don't fix it. https://www.nespresso.com/il/he/

More than once I have spent precious time filling out some form or going through some wizard only to find that I cannot “submit” or that validation is failing on some field or other in Firefox. Switching to a Chromium based browser works. I know it has happened to me at my bank. I cannot recall exactly where else. Reservation systems? A couple of travel approval sites I think ( during COVID ).