1. Go to `about:config` and convince it that you know what you're doing.
2. Search for `media.autoplay.default`.
3. Double-click `0` and enter `1` if you want to always block, or `2` if you want to be prompted for every site that tries to autoplay (which can help with some sites that don't work quite right with blocked autoplay.)
4. Enjoy your significantly-more-pleasant browsing experience!