Earlier quoted context omitted.
On my case it works when I force enable it, but it leads to occasional X crashes.
You can force enable hardware accelerated video encoding? How? And what difference do you notice when you watch Youtube videos?
Check "How to force-enable blocked graphics features".
https://support.mozilla.org/en-US/kb/upgrade-graphics-driver...
https://wiki.mozilla.org/Blocklisting/Blocked_Graphics_Drive...
The difference is that my tiny netbook APU doesn't get bogged at 100% while playing videos, on an otherwise DX 11 class GPU.