I'm aware of a few things, myself:

1) Google properties

1a) Chromecast

2) a few web-based games that were really pushing the envelope on web APIs and didn't bother testing on Firefox

3) WebUSB, commonly used for some things like keyboard customization apps

A lot of IT now curates the extensions for the browsers and doesn't allow extensions not on the whitelist and then they basically just only do that work on Chrome and disable Firefox. It's kinda self defeating in the long run imo but that's the problem in the industry.

I've run into a few restaurant sites whose ordering pages just do not work properly (or at all) in Firefox. Also webgl2 performance is unfortunately still much better in Chrome vs Firefox; as an example, FoundryVTT (virtual tabletop software) works fine in Firefox but is a stuttery mess IME (though it has improved slightly in the last few years).

Chrome likes to make up new "standards" and then some websites adopt them immediately.

That said, I can only remember two instances of that slightly inconveniencing me in the past, and both times I was inconvenienced by a Google-run website: once upon a time Google Earth refused to work, and once upon a time I couldn't tweak my Google Meet background. Both are no longer the case.

The driver and store signup/portal for doordash returns a 403 forbidden on firefox.

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.