I tested the issue that you describe and I can not reproduce it using the last version of the Telerik controls and FireFox 4. Please take a look at the attached image. Can you reproduce the issue on our on-line demo here or it occurs on a localhost only?
We tested the online demos in several others machines with firefox 4 but we can not reproduce the issue. How does this demo appear in other browsers (IE, Safari, Opera, Chrome)? Since we have not had such a complain so far can you please check and let us know?