It depends on whether sites use a plugin for video. Some kinds of video can be played by Firefox directly. That uses the built-in HTML5 video player, in combination with the Windows Media Foundation (to decode MP4/H.264 video). There are two hidden settings that help control when videos start which you could test out to see whether you like the results: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful. (2) In the search box above the list, type or paste '''media.''' and pause while the list is filtered (3) ''To avoid videos in background tabs starting before you activate the tab:'' Double-click the '''media.block-play-until-visible''' preference to switch the value from false to true (4) ''To pause the player on sites that activate it automatically:'' Double-click the '''media.autoplay.enabled''' preference to switch the value from true to false -- note that the video will still buffer, this doesn't block downloading, just pauses playback. Unless it was fixed by now, Firefox doesn't update the controls on the player, so the video may show a pause button instead of a play button. In that case, you need to click the pause button, then the play button, if you actually want the video to play.