Rated 5 out of 5 stars
Hello. Your addon works flawlessly on my FF 13.0.1 and I pretty much like it.
The only thing it lacks, imho, is the ability to cancel not only autoplay, but also automatic video pre-loading.
I personally prefer saving most youtube videos with a download manager and then watching, so I always have to right-click on youtube player and select "stop download".
Download 1.0.0 from https://addons.mozilla.org/firefox/addon/stop-youtube-autoplay/versions/
(it's awaiting review)
Go to the options, and check "Do not buffer videos"