There was a browser compatibility problem with Firefox 6 in Sitefinity 4.2 1650, which is resolved by applying Sitefinity_4.2_Hotfix_1. You can find it in your Clients accounts -> Download and Manage your products -> Download (under Your trial products) or Download Installer and Other Resources (under Your Purchased Products).
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