Annoying if you have a lots of tabs open and you startup Firefox and all your videos starts to play at once. Or just that one tab that you can't identify, because it's not Youtube, and you don't recognize the icon.
Here is the tip that will solve this big problem.
Type in the address bar:
If you have the warning, ignore it, and just go on.
Search for autoplay, and you'll find:
You can modify the Value (last column) three different ways:
0: Autoplay default, this is the default value.
1: Autoplay blocked for all sites.
2: Ask the user. If it comes to a new site where there is a video/audio source. It will ask you if you want autoplay on this site.