I think it's because since forum is https, browser blocks all http content. If i disable mixed active content blocking in firefox, i can see all embeded videos without problems here on forum.
I think you should search for youtube plugin, that can work with https links. This seems can do it:
http://www.simplemachines.org/community/index.php?topic=436292.0I'm no expert, so i can talk rubbish :]
It would be very nice to get embedding working again.
