Try to disable the addon, and you will see that other videos on YouTube will
still work with HTML5 (placebo effect). The video in question is encoded in
WebM, and should work, but there are certain problems with YouTube, that
prevents this.
Regardless, you can still stumble upon some videos that are not encoded in
WebM. To check whether a video is available in WebM (and thus HTML5), you can
use a free online service called KeepVid, that will reveal you the direct
links, where the video files are stored.
I worked on this topic in the past, and already solved it (accidentally). I
can post here a workaround if you want.
