BNP bank site rarely but regularly breaks on FF (just shows white screen). At least one payment processor had disabled input fields, so not working. At least two railway portlas in two different countries effectively didn't work for some time because station dropdowns were broken and you couldn't proceed next without selecting. Just today I found a bug in Jira, when external integrations don't work on FF. Stadia doesn't work on FF by design. Skype web doesn't work on FF by design. Streaming services block streaming high def stream to FF because it lacks DRM unlike Chrome clones.
All of this I saw in the last few years, some of that is still valid today.