Firefox html5 video support

How can I get Firefox to fully support html5 video?

The YouTube HTML5 Video Player says that H.264 and Media Source Extensions are not supported. Is there any way I can enable all of these?

Many of the features on that table are extremely new. The Media Source Extension spec, for example, is still a draft and is still being written. And it's being driven by Google and Netflix. It's going to take time for the spec to mature to a point where Mozilla want to spend any time implementing it.

h.264 support is somewhat easier (if you're on 12.04 - it's currently muddier if you're on 14.04):

sudo apt-get install ubuntu-restricted-extras

Update: In Firefox 30 (should be the default in all supported versions now), the feature is there but it's disabled by default. This should change in Firefox 31 but you can enable MSE by going to about:config (stick that in the address bar) and searching for mediasource. That should leave you with the media.mediasource.enabled option. Double click to enable.

On the Youtube panel the MSE & H.264 option is still crossed out... But the other two MSE options are now green.

On top of the accepted answer, on Ubuntu 14.10 (FF 36.0) to obtain the OK for the MSE & H.264 you should set this properties to true on about:config:

• media.fragmented-mp4.exposed=true
• media.fragmented-mp4.ffmpeg.enabled=true
• media.mediasource.mp4.enabled=true

Here you can find more details