Watch YouTube in the full-HD quality or the highest available oneHD quality for YouTube™ 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 through the gear icon beneath each player
The extension also allows user to select lower HD qualities as well. Please visit options page for more details.
There is a FAQ section in addon's page as well,
If you have a question you can also open a bug report on the GitHub page,
As of version 0.2.2, it is possible to set video quality to not HD qualities as well.
Reviews of HD quality for YouTube™ (1080p, 1440p, 2160p) extension