Works perfectly with a few other tweaks!

When I first installed this add-on, it worked perfectly. I recently uninstalled it when YouTube updated it's platform.

This is how I got it to work again and how I stopped YouTube videos from preloading, AT ALL.

1. Get this other add-on YouTube Design Preserver 2.2.0 https://github.com/GuyXY/YouTube-Design-Preserver. Set it to 'Old Layout'

2. In your web browser, go to About:Config. Scroll down to and double click media.autoplay.enabled. This will change the value from 'True' to 'False'. No media on any site is allowed to start playing automatically.