Why is such a feature even needed?
Some like me, when stumbled upon an awesome song(like Handlebars by Flobots or Viva la Vida by Coldplay), would really want to hear it more than once. And in YouTube without an Auto replay button, its really a pain.
I had bookmarked the YouTube Chromeless JavaScript API reference long time back that came real handy today. I was able to finish the add-on in no time with these simple APIs.
Install your copy from here. As always, report any bugs/issues here. Feature requests are always welcome!
ps: You don't need a mozilla account anymore to install the experimental add-ons. You just need to check the "Let me install this experimental add-on" checkbox and carry on with your installation.
Update 0.0.4: Added options to choose the portion of the video to be auto replayed. [Recommended install]