YouTube™ HD Plus
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!