Just a hit, to save you from using time on this problem as I just did. I don't know if this is something to do with Silverlight 3, I can just say that I have never experienced this problem before.
I'm currently working the game Little Longhorn and have just upgraded it to Silverlight 3. Beside not understanding why they removed the asp:Silverlight control, it went pretty painless. Well, untill I opened up my game in FireFox 3 (3.5). In FF my game where gone! It never appeared. I could see it in the source of the site, but it never showed up. After pretty much time comparing to an "clean" silverlight 3 application, I discovered that I needed the style below:
Actually it were the height that were missing. Apparently FF don't give the surrounding div an height. Just a little FYI if you get into this problem.
Hope it helps.