Works for the most part. My configuration is (Windows 7 Home Premium + Latest version of Firefox).

When an YouTube Video page is being loaded, the video is successfully blocked from playing automatically.

However, the downside is, once we click the red box (that this add-on overlays on top of the video) and make the video play, the add-on does not seem to work when we click and open a "suggested video" that is displayed on the original video's page. Hopefully the developer could sort this out. I will increase the Star Rating to 5 once this issue is taken care of.

Steps to recreate issue:
1) Load any video on YouTube
2) Click the red box displayed on top of the video to make the video to play
3) Now click one of the suggested videos displayed on the right side of the original video's YouTube page (Or, after the original video you opened completes playing, click on one of the suggested videos displayed on the Video Player).
4) The suggested video will load, but this time the add-on fails to kick in. This video still automatically plays.

Workaround for the time being:
In step #3 above, instead of directly clicking on any of the suggested videos, open the suggested video in a NEW TAB (by right clicking and selecting 'Open link in new tab' or by pressing and holding the 'Control' key in your keyboard while clicking). Now the add-on kicks-in in the new tab as expected and stops the auto play of the video in the newly opened tab (even if the video on the original tab is still playing).