It's a very annoying and quite a drastic bug, since it kills all open firefox tabs stone dead (e..g the bug report you're tying into channel 9 forums...), not just the msdn page. The workaround from Adam777 of clicking the popup in another browser works as follows:
* Launch IE (or other browser I suppose, I haven't tried) * Navigate to the video in this browser * Start the video, request fullscreen * On the confirmation popup, check "remember my answer" and then click the "Yes" button. * Restart firefox * Navigate to the video in firefox * Firefox will now fullscreen the video without using the dialog of doom.
You can then recreate the bug again by de-remembering the answer: right-clicking the silverlight video, selecting the "silverlight" menu item to bring up the "Microsoft Silverlight Configuration" dialog, go to the Permissions" tab, and remove the permission for "http://channel9.msdm.com:80 | Stay Full-screen when unfocussed"
The dialog of doom will return.
Perhaps people who cannot replicate this problem could try this?