Embedded YouTube videos not working

Just to make it more confusing: I lost my ability to watch ANY youtube video (embedded or directly from youtube.com) the precise moment I upgraded from 10.5.8 (Leopard) to 10.6.x (Snow Leopard), but ONLY on my (Mozilla-based) SeaMonkey browser. Safari works fine, even before I updated Adobe Flash (which I did). It's possible that ONE of the plugins in my SeaMonkey browser is to blame, but I don't want to go through the trouble of disabling them all without knowing WHICH one. (MS Silverlight? One of the way too many Java plugins? RealPlayer???) The confusing thing is it all worked under 10.5.8 (I've updated to the latest, 10.6.4, by now).
It may be paranoia, but I saw a discussion in Google about the new HTML5 format for embedded video. Apparently Apple and Google (youtube) are going with a proprietary protocol which Mozilla refuses to accept. Go figure...
http://www.google.com/support/forum/p/youtube/thread?tid=0aa3175f8fbb924a&hl=en

Oh wow. After all the things I had tried, including removing all plug-ins from ~/Library/Internet Plug-Ins and /Library/Internet Plug-Ins, I disbelievingly did the Cookies->YouTube->Remove trick and boom, suddenly embedded YouTube clips work. Once again, wow. Thank you for resolving this months old problem for me!

I kinda wish now I had kept the list of cookies - I wonder what the culprit actually was!

It looks like a Flash issue. And this is why Apple has been discouraging the use of Flash. It's very buggy, and people are inclined to blame the Mac or OS X when it's the fault of third party software.