Take a look here
at some old documentation that was written when WebAii 2.0 was released. Look at the Configuring Your Browser > Firefox section; it goes up to Firefox 3.5.
Have you checked out our 2011 version of the Framework? It includes support for FF4 and WPF 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?