We'll fix the issue and will introduce the FF6 support with the first service pack that we release for Sitefinity 4.2. Firefox 5 is working fine though. We'll announce the date for the service pack soon.
Thanks for your patience!
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items