ups.com is one that really infuriates me. It shows 404s for me on Firefox and works perfectly on Chrome.

Kaiser's website works mostly on Firefox. Recently I had to print a "letter" and on Firefox it was blank and printed fine with Chrome.

I don't know if it's still this way, but Google Meet didn't work very well in Firefox, so last year I took all my meetings in Chrome.

These are just what I remember. There are a LOT more.

EDIT: on the UPS thing... it happens when I follow links from gmail in Firefox. Sometimes it wouldn't 404, but I'd see a "..." and it would just stay that way.

EDIT2: for a long time (not anymore), sending Kaiser emails was broken. Hitting enter would warp to the bottom on the page and I'd have to scroll back up to finish typing. They're completely redesigned the website recently and that bug is fixed.