[user5994461]

You don't need an extension for that. Go to about:config and set media.autoplay.enabled to false.

[nasredin]

Spent a lot of time on this, so:

NoScript ("allow first-party scripts" and block everything in "Embeddings" tab) + FlashBlock seem to be the only way to block all autoplaying crap in Firefox for me.

You can also add uMatrix, uBlock & RequestPolicy and surprisingly they don't conflict with each other!