From Hack Sphere Labs Wiki
Enable HTML5 Video Firefox
Enable MSE & H.264 A new preference in Firefox Nightly 34 changes that however, so that support for all requested technologies is provided afterwards on YouTube. Note: While the article concentrates on Google's video hosting platform, enabling support for MSE & H.264 will benefit users of the browser on other websites as well. You need to create a new preference to do so: Type about:config and hit enter. Confirm you will be careful. Right-click and select New > Boolean. Name the preference media.mediasource.ignore_codecs. Set its value to True. Update: Linux userrs may change the following preferences as well: media.mediasource.mp4.enabled to true media.fragmented-mp4.* to true media.fragmented-mp4.use-blank-decoder to false If you go back to YouTube's HTML page, you should see all six technologies listed as supported (in green).