Firefox 4 currently is in RC status, which means it's not (yet) officially supported by us. However we're hoping/expecting that our existing Firefox add-on will work unchanged. We just haven't had the chance to verify this yet since the RC was just released a couple of days ago.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!