http://www.pcworld.com/article/2922...ml5-videos.html

Firefox

You don’t need an add-on or an extension. You can take care of this with a setting in Firefox itself.

Open a new Firefox window or tab, and in the address field (where you would normally type a URL), enter about:config. Then Click the scary I’ll be careful, I promise button.

You’ll get a long list of settings, and above them all, a search field. In that search field, type autoplay and press Enter. This will bring up one entry: media.autoplay.enabled.

Note the Value column. It will probably be True. Double-click the entry and the Value will change to False.

The next time you visit a page with an HTML5 autoplaying video, the video box will appear with an image in it. But the video won’t start until you tell it to.