I got tired of Youtube Center reseting the settings. I tried to find a script that changes the video quality. All of them were quite bad. Some didn't work at all and some were just awful and the code contained comments like "don't know what this does. just copied it from youtube center".
So I made my own with the help of the almost working scripts. This script has only been tested on Firefox 35 on 28th of January 2015. If this doesn't work for you, leave a comment. I'll see what I can do.