Firefox 6/7 was introduced with some critical changes in its core which affected the functionality of Sitefinity. Can you specify which version of Sitefinity you are using? If it is 4.2 ( not the SP1 ) you will most probably be getting such behaviour while browsing with Firefox 6+. However as Nicolas pointed the fix for the problem was included in SP1. If you do not wish to upgrade, please download the patch that we have prepared for 4.2 1650. You can find the archive in your Account -> Downloads page.
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