If you're looking for an alternative to YouTube High Definition add-on this is the one.

With a lot of features that you didn't know you needed this is one of the best things out here, making YouTube actually a lot better than its current state.

This powerful add-on allows you to:
• Set a fixed resolution for video playing so you don't have to change it every time.
• Always display subtitles when available.
• Add a loop button so you don't have to right-click to get the menu for loop. Loop can be active for ever.
• Change the video player size and features.
• Apply a visual theme for YouTube background, auto-expand descriptions, hide ads and much more!

Not working on mobile view? Toggle Firefox desktop mode and you'll be ready to go.
This is a must have!