Scott Hanselman has posted an excellent little example on creating an XBAP that runs in Firefox / IE using .NET 3.5. What is surprising is how little code you actually need to do some pretty complicated things because the "heavy work" has been done for you.
The link is http://www.hanselman.com/blog/FirefoxWPFAndXBAP.aspx.
Deja View - The feeling you've seen this post before.