Your approach sounds workable. I did a little testing on my own and successfully navigated from an HTTPS non-Silverlight website to an HTTP Silverlight website in Firefox 4.
Is your website publicly accessible? Can we access it to study this problem? Can you send us the exported test results (see attached screenshot)? Can you record a video of this happening for us to review? Jing is a decent free screen recorder.
the Telerik team
for a live 'What's New in Test Studio R1 2011 SP2' event on Tuesday, July 19 at 2pm EST!
Have you looked at the new Online User Guide
for Telerik Test Studio?