I believe its some time in this year, the HD quality options, thats 720p and 1080p, were missing, and not only those, actually I only got 360p of all options.

Currently, as of 2015-12-05, I am using Firefox ESR 38.4.0. In fact, I have found the solution much earlier even though this official YouTube Help gives absolutely zero help at all but never bothered writing it, but now I have decided to write a post.

From what I can check with about:config.

If I turn on:

  • media.fragmented-mp4.exposed
  • media.fragmented-mp4.ffmpeg.enabled

I will get 720p HD with video/mp4; codecs="avc1.64001F, mp4a.40.2". Thats just 360p and 720p for now.

If I also turn on:

  • media.mediasource.enabled

I will also get 1080p HD, 144p, 240p, and 480p. The 720p is video/mp4; codecs="avc1.4d401f". Now its the full set as before.