I´m using Firefox Developer Edition on Windows. With one of the updates within version 37 youtube videos would not play anymore. I am not sure whether this is due to youtube´s permanent switch to HTML video or to one of the updates. In case someone stumbles over similar issues you can check at youtube.com/html5 what video codecs can be played by your browser. In case you are missing „MSE & WebM VP9“ (see picture) as I did you can tell Firefox to enable that one.
- Enter „about:config“ and confirm you will be careful
- Enter media.mediasource.webm.enabled
- Set the value to true
After reloading the youtube HTML-video page all codecs should be available. In case you are missing other codecs search for media.mediasource and try to set these values to true as well.