On Firefox set media.autoplay.enabled to false under about:config. As of version 41 this blocks play() unless you've interacted with the video. https://bugzilla

On Firefox set media.autoplay.enabled to false under about:config. As of version 41 this blocks play() unless you've interacted with the video. https://bugzilla.mozilla.org/show_bug.cgi?id=659285 https://bugzilla.mozilla.org/show_bug.cgi?id=659285 You can also set plugins.click_to_play to true to do the same for Flash.