It didn't work - I opened one video page and it started playing immediately. I put it on pause and the load bar continued moving, so even on pause it was still loading. I have a data limit on my connection; auto-play is NOT an option for me. (This is my first attempt to use an up-to-date version of Firefox, and I've had to change almost all of my addons. If I can't find something to stop YT - or any - videos from auto-playing I'll be abandoning this version and going back to the one that works.)