While Google Chrome browser uses HTML5 player on the YouTube site for playing videos whenever possible, but that’s not the case with Firefox, the default player currently supports HTMLVideoElement, H.264, WebM VP8 and good news for Firefox users using Nightly build that is currently on version 32 is support for Media Source Extensions for HMTL5 player is hidden behind a preference which you can enable and enjoy better YouTube Player, here is how to do that.
Enabling Media Source Extensions in Firefox
1. Visit about:config
2. Double click on "media.mediasource.enabled" preference to turn to true.
Now when you visit YouTube.com/html5 in Firefox, you can see the default player supporting all except MSE & H.264.