I've read an article about How to Stop Auto-Playing HTML5 Videos in Your Web Browser at http://www.howtogeek.com/227669/how-to-stop-auto-playing-html5-videos-in-your-web-browser/ which states, concerning Firefox :
To Mozilla’s credit, Firefox actually contains a preference that allows you to control whether HTML5 videos on web pages automatically play or not. However, like many Firefox preferences, this one is buried deep in about:config where you’d otherwise never find it.
Mozilla improved this preference in Firefox 41, making it more powerful. 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. A script running in the background can’t just instruct the video to play without your permission.
To change this setting, plug about:config into Firefox’s address bar and press Enter. Agree to the warning and then type “autoplay” into the search box. You’ll see a preference named “media.autoplay.enabled”, which will be set to True. Double-click that preference and it will change to False.
Hence, I've been searching unsuccessfully for a simple add-on which would include a toolbar button just to toggle this media.autoplay.enabled setting.
Does anyone happen to know if such an add-on is available? Otherwise, maybe an interesting idea for a coder?
Thanks!