It sometimes failed to load reaction icons on Linkedin. This was happening for a long time, but seems that either Firefox or Linkedin fixed it. I didn't stumble upon it recently.
Firefox don't play MKV videos, I use some site that has embed MKVs.
Slack calls didn't work on it (they workaround it). But I had trouble with other sites that use WebRTC in Firefox.
This is things I recall at this moment, but I don't want to have to use backup browser when something like this happens, so I use Chromium based browser and get on with my life.
