works for OLD YT videos, NOT for new ones (based on HTML5) Rated 2 out of 5 stars

.. the title should have made it (finally) clear ....

for details see Kelvin's comment below https://addons.mozilla.org/en-US/firefox/addon/youtube-autoplay-stopper/reviews/331581

FF 10.0 Windows, Youtube AutoPlay Stopper 1.0.7

This user has a previous review of this add-on.