HD quality not available in Youtube videos on Firefox

Problem

I was trying to watch the trailer for the upcoming Star Wars movie on Youtube. I noticed that I could get only a maximum of 360p quality video. None of the HD quality options were shown in the video settings!

Solution

I have configured my Youtube to play using HTML5, not Flash. I went to the Youtube HTML5 video player webpage and it showed the status of my browser as:

There are many items that are shown as unsuppported. However, the key item here is H.264, which is required for HD videos on Youtube.

I opened about:config in Firefox and searched for webm. The option media.mediasource.webm.enabled was set to false. Double-click it to set it to true.

Restarted Firefox and checked the HTML5 player status webpage. H.264 now appeared as supported. And my Star Wars trailer played in HD! 😄

Tried with: Firefox 40.0.3 and Ubuntu 14.04