YouTube videos are programmed to play only once by default but what if you wanted to repeat a video over and over again in YouTube?
Once you have installed the script the video will automatically replay over and over again once you click the Loop link without you have to use the replay button.
YouTube has a inbuilt functionality to repeat videos which publishers can use while embedding the videos on their own site. To make the videos repeat over and over again you will need to use the key value pair loop=1.
You can check the entire list of parameters supported by the YouTube player.
Note: You will have to first install the Greasemonkey extension and restart Firefox once before being able to use this feature.
Via: [Google OS]