Skype Web is an example from the top of my head, but I don't run into them a lot either (or don't notice that they are only broken in Firefox, because I rarely care enough to try another browser).
I do see quite a few sites that apparently never have been tested with ublock or privacy badger, but they break on all browsers where those are enabled.
I do see quite a few sites that apparently never have been tested with ublock or privacy badger, but they break on all browsers where those are enabled.