Videos shouldn't be auto playing when embedded in a
thread. When you visit the actual Youtube page, the video will autoplay. There is a setting in Firefox that can turn off certain media types from playing automatically.
Open a tab on Firefox and in the URL type: about:config
You'll get a neat message about messing things up. Of course, we're going to be careful. Press on.
In the search bar at the top of the screen, type: autoplay
You'll get one line that reads: media.autoplay.enabled
Double click on that line and it should read "False" for the value.
Close the tab and you're golden.