I'm afraid this isn't actually a bug. Mozilla radically changed the browser window in Firefox 4 (and 5). The ContentWindow property in Firefox now returns the same Windows window as the "window" property. Firefox 4+ no longer has a separate Win32 windows for its chrome and its content. They are now one in the same window. I'm afraid you'll need to account for this difference in your testing.
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?