Prevent Firefox browser from autoplaying YouTube HTML5 videos. Thanks to click-to-play, which blocks flash videos from autoplaying in Firefox and Chrome browsers, now we need to find a solution for HTML5 videos as well.
Have you annoyed with certain (HTML5) videos being automatically played in websites like YouTube or Facebook news feed, you can stop that from happening in Firefox browser natively through a preference.
Stop HTML5 videos from auto-playing in Firefox
1. Visit about:config,
2. Find the preference media.autoplay.enabled using search and toggle its value to False.
Note: Let me tell you this setting will be honored only in Firefox 41 or later versions, but not in the current Firefox 38. You can install Flash Control or No Script add-ons to achieve the same in stable or any other version of Firefox.