It doesn’t support a lot of video formats that Chrome and Safari have supported for years (h265 is one I think. I’m no expert)
I don't event think h265 is widely supported. On Windows you have to pay separately for it
Firefox has had support for h265 for a few months by now, they finally relented.
h264 and h265 are patent-encumbered and therefore very expensive and/or dangerous. Patent trolls would rip Mozilla apart and eat all their money. The only reason H.264 works atm is that Cisco sponsors a plugin for that.