The Source code of this KISS (Keep It Simple, Silly!) PHP project: Click here!
Explanation: We just use Youtube API Player parameters about this loop, and the trick is we to create "playlist": http://www.youtube.com/v/VIDEO_ID?autoplay=1&loop=1&playlist=VIDEO_ID
Then there is a loop! And that's all! You can write the URL/Link in the Address Bar in your favorite Browser and will work too. Just change this VIDEO_ID ... I know it's not comfortable by this way:) When there is a playlist, then Youtube's player load next video automatically, and when we create two same videos, well there is Loop/Repeat mode. Look at the top left corner in Youtube player here to see the playlist with 2 same videos:)
You do not need to use any addons buttons and install them in your browser:) just no need of this:)
You can use my free web hosting and to upload this script there: www.Zoho.cf - Free Web Hosting