ATTENTION: The currently reviewed version of this add-on no longer works. A fixed version, 1.0.3, is awaiting review by Mozilla. In the meantime, you can install it by going to https://addons.mozilla.org/firefox/addon/stop-youtube-autoplay/versions/
The add-on no longer works with Html5 videos due to a YouTube change. I am working on a fix, but it might take a while. Please check if you are in the Html5 trial before reporting problems (right click on the video, and if the last menu item says About HTML5, then you're viewing an Html5 video, you can disable it by going to http://www.youtube.com/html5).
This add-on prevents YouTube videos from auto-playing.
It allows videos in playlists to auto-play after the first one, so you can play all of them without having to start each one manually (this can be disabled).
It can also be configured to only prevent autoplay in background tabs, and to start playing a video when you switch to a background tab.
Please use this thread for bug reports and support: https://forums.mozilla.org/addons/viewtopic.php?f=23&t=10938