It doesn’t seem possible to do this on Apple’s Safari web browser. The same Disable HTML5 Autoplay extension you can use on Chrome is also available for Opera. Opera is a Chromium-based browser, just like Google Chrome, and supports the same browser extensions. Double-click that preference and it will change to False. You’ll see a preference named “”, which will be set to True.
Agree to the warning and then type “autoplay” into the search box. To change this setting, plug about:config into Firefox’s address bar and press Enter. A script running in the background can’t just instruct the video to play without your permission. When you disable autoplay of HTML5 media, scripts running on the current web page won’t be able to start media unless you’re interacting with it.
Mozilla improved this preference in Firefox 41, making it more powerful. However, like many Firefox preferences, this one is buried deep in about:config where you’d otherwise never find it. To Mozilla’s credit, Firefox actually contains a preference that allows you to control whether HTML5 videos on web pages automatically play or not. Of course, this same add-on will also work in Chromium. This one doesn’t have as many users, but promises to disable autoplay in all situations - including blocking scripts from automatically playing videos and parsing new HTML5 videos as they’re dynamically loaded on web pages. If this extension doesn’t seem to work for some reason, you may want to try the Disable HTML5 Autoplay extension.