Not many of you will have GreaseMonkey script installed on your system and this is for you people. There is a new Firefox Add-On, that adds infinite replay loop functionality to YouTube videos.
This is still an experimentail add-on. You can install this as in with no conditions what so ever.
The infinite loop feature is not available for playlists yet.
Related Post: Did you know that you can change youtube video position / order in Playlist.