YouTube™ HD Plus
By lack of time YouTube™ HD Plus hasn't been updated since January 23, 2016 so I highly recommend you to use Enhancer for YouTube™ instead.
By default YouTube™ serves videos at a low definition to users that don't have a large monitor, even if they have a fast internet connection. This is probably to save a huge amount of bandwidth but, as you may have already noticed, manually selecting your preferred quality for each video is really annoying, especially when you want to watch a lot of videos in HD.
To easily solve this annoyance you can use YouTube™ HD Plus if you don't need all the features that YouTube™ Enhancer Plus brings.
YouTube™ HD Plus is a very lightweight and restartless add-on that automatically sets your preferred quality, whether you want videos to start playing in HD or at a lowest definition if your ISP imposes bandwidth caps on its customers.
Your preferred quality will also be applied to embedded videos, not just videos hosted on YouTube™, and since this add-on also allows you to stop autoplay for videos loaded in background tabs, your overall YouTube™ experience will be significantly improved.
So if you're tired to click the little gear icon over and over each time that a video is loaded, click the button below to freely install YouTube™ HD Plus directly from Mozilla's website and let it perform this repetitive task for you!