Sorry for any inconvenience caused by these videos.

First off, most websites like Facebook have a setting to disable automatic video playback. You will need to contact the website's support team for more information on these settings. For example, you can disable Facebook video playback by changing the video settings on your account.

Although there is no builtin Firefox feature to disable automatic playback, there are a variety of addons that you can install to achieve this. You can try an addon like Flash Control or FlashStopper to block Flash video playback, as that is a common form of video content. However, HTML5 is getting quite popular, so you may need to install a separate addon to block those, depending on the addon you install. Unfortunately, I am not able to advise you as to which addons work best, nor can I ensure that these addons will work in all situations. However, this is a good place to start.

Adblock Plus {web link} Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

Adblock Plus Pop-up Addon {web link} Adblock Plus Pop-up Addon extends the blocking functionality of Adblock Plus to those annoying pop-up windows that open on mouse clicks and other user actions.

Forum; Adblock Plus Homepage {web link}