YouTube HD is a very lightweight extension that forces YouTube's HTML5 and Flash players to play videos in full-HD or highest available quality. Exactly similar to changing the quality from the gear icon
Note: It might take up to one second for the player to change the quality to HD. This helps the player to start your video as soon as possible and fetch data required for playing HD.
To report a bug please go to https://github.com/james-fray/YouTube-HD/issues
To see FAQ page please visit http://add0n.com/youtube-hd.html