A simple auto-HD script, which does not re-format the page as YouTube Center does. Very fast.
All of these features can be disabled with some console commands. Go to a YouTube page, open the console, and settings are stored in an HDOSettings object. It has properties:
HighestQuality(number from 1 to 9)
Change these as you like, and run
SaveHDOSettings() in the console to save your settings. See screenshot for help.
For example, this code will set the desired quality, enable autoplay and save changes:
HDOSettings.HighestQuality = 5
HDOSettings.VideoAutoPlay = true
Default is 1080p (6).
HighestQuality to a number between 1 and 9 accordingly: