In old versions of Firefox, preventing videos from autoplaying required an extension such as the now-defunct Flashblock. More recent versions of the browser have an "Ask to Activate" feature for plugins, which stops Flash videos from loading automatically -- and since Firefox 30, the browser defaults to disabling plugins until needed. "Ask to Activate" does not apply to HTML5 videos, but you can stop these unwanted clips from playing by also turning off "media.autoplay.enabled" on the about:config page.
Open the Firefox menu and click "Add-ons" to open the Add-ons Manager.
Switch to the Plugins tab and scroll down to Shockwave Flash. Open the drop-down menu for Flash and choose "Ask to Activate." If you have Silverlight installed, set it to "Ask to Activate" as well.
Enter "about:config" without quotation marks in the address bar and press "Enter." On the warning page, click "I'll be careful, I promise!" to access Firefox's hidden settings.
Search for "autoplay" on the config page, and double-click "media.autoplay.enabled" to set its value to "false." This option stops embedded HTML5 videos from playing automatically, but does not block videos you browse to manually, such as those on YouTube.
Click on a Flash or Silverlight video to activate a plugin when necessary. After clicking the video, choose "Allow Now" to run the plugin once. To let the current website use the plugin at any time, choose "Allow and Remember" instead.