The linux version of firefox still doesn't even have opengl compositing by default, so I wouldn't hold your breath for hardware decoding any time soon.
Not sure if this is related but my firefox/linux experience with video is also bad:
- tearing all the time (https://en.wikipedia.org/wiki/Screen_tearing)
- Netflix doesn't support HD on linux
- Amazon doesn't support HD on linux (for movies. But shows seem to be 1080p)