I had this problem for sometime, have gone to Mozilla and gotten little for the trouble.
Anytime I attempt to load a Youtube file from a link in Thunderbird, Firefox loads, loads the video and then promptly crashes at the point where the video is going to begin to play. I can copy the link and attempt to run it in Firefox and Firefox still crashes. I can go to Youtube, in Firefox, and play the video just fine. So, its something about the embedded video that Firefox does not like.
I can open SeaMonkey and do a Copy and Paste from Thunderbird and the video plays just fine.
The above is true for a rather large percentage of embedded videos in Thunderbird that I try to open with Firefox.
The Seamonkey is a work around and I would used Seamonkey and delete Firefox from the system, if Seamonkey were better at displaying attached files on e-mail.
So, any clues?
My video is a Nvidia 9600 with 1gig ram.