If you want to try it out, you can enable "MSE & WebM VP9" on the "about:config"-page (Just type it in the titlebar / adressbar without the "").Search for "webm" and it should come up with 4 results - double click on "media.mediasource.webm.enabled" to change it from "false" to "true".Now check the the Youtube HTML5 Page again - everything should be enabled now and therefore every feature should work on Youtube - well, I only can say that it works for me till now without any problems or bugs :-)
另外，记得在FlashBlock中把Youtube加入whitelist。（也可以uncheck "block html5 as well" )
