Youtube stopper + Security Rated 5 out of 5 stars

Not as much of a hassle as I thought it would be, and plus is it's a good youtube stopper, regardless of whether the video is in HTML5 or not.

A better interface for Firefox's obscure click-to-play ability. Plugins can (though rarely) be a security risk if run, so modern browsers appear to be giving users better control on this.

Reposting helpful hints:
If it doesn't look like it's doing anything, set individual plugin settings to 'Ask to Activate' in your extension manager.
If you can't see a plugin click-to-play interface, you can toggle the little plugin icon that appears in the urlbar.