I'm no longer going to be updating TubeStop, a Firefox extension I wrote that disables autoplay on YouTube videos. I don't have the time or the inclination to keep up with YouTube's HTML changes.
TubeStop has been around for about five years, and it has 17,000 users. It was the first browser extension that made it possible to disable YouTube's autoplay feature (if you don't count the all-purpose Flashblock), and it gained notoriety in 2007 for inadvertently stripping ads from YouTube videos.
If you'd like to adopt this abandoned add-on, let me know, and I'll transfer ownership of the extension to you on Mozilla Add-ons. If nobody wants to take over development, I'll shut it down and the void will be filled by one of the other anti-autoplay extensions on AMO.
Update: TubeStop was discontinued on December 25, 2012.