The latest version of Firefox doesn't play too nicely with YouTube browser add-ons that affect the way YouTube works and plays videos. I typically use a browser add-on to prevent YouTube pages from playing automatically because I tend to load many YouTube tabs all at once (imagine having an entire chorus of videos play all at once).
To stop autoplay completely on all YouTube pages in Firefox, do the following:
- type about:config into your URL bar.
- Click the button to accept the risk.
- On the next page, type media.autoplay.enabled
- Double click on this item and it will change to false.