I’m getting this here. Every time I've tried, for several weeks.
To replicate, I do the following: Open firefox, go to a channel 9 video, start the video playing, click the “fullscreen” button. “The do you want to allow the website to say in fullscreen mode” dialog comes up, but it's not resposive to clicks and that’s it – firefox has crashed.
I suspect that you my not get this if you previously managed to check the “always allow fullscreen” setting on the popup. If you haven’t, well, Silverlight crashes firefox hard. The whole FF locks up, not just the one tab, and has to be killed via task manager. I have this across the last 2 released versions of Firefox – 3.6.10 and 3.6.11. This only happens on channel nine videos.
I have windows 7 and Silverlight Version: 4.0.50917.0