This extension is no longer needed. Firefox already includes a setting to block media autoplay. Go to settings and search for "autoplay". Click on "Autoplay → Settings...". I suggest changing the default for all websites to "Block audio and video".

Firefox also makes it super easy to add exceptions for certain sites. Just click on the Permissions icon to the left of the address bar and it'll give you a quick drop down menu to permanently allow auto playing both audio and video or only audio.

Of course, although it is no longer necessary, I thank the author who kept this extension updated all the way until 2018. You helped a lot of us out,