They're hidden behind an about:config flag - media.mediasource.enabled
The latest version of developer edition/aurora just enabled them by default, so version 36. They may be only ~12 weeks away from becoming default in the stable build.
Thanks, I didn't know that setting already made it into the stable build. Now I can watch Youtube in high resolution (and in VP9 with that!) without using Flash. Why is it still disabled though, there are some bugs left?
