It does indeed stop autoplay and pre-loading and at first appears to be an excellent solution, but unfortunately it also interferes with YouTube's ad system in such a way that the ad cycles in a loop and the video never starts. That makes it unusable for me. I tested this in Firefox 65.0.2 on Windows7-64.

Stopped working in the last few days.

Not working in embedded YouTube in twitter ps fix video auto playing fix it ff version 54.0