I was unable to reproduce this problem in a sample application and also just replied your ticket on this reported issue.
Are you also able to reproduce this in our online examples here: http://www.telerik.com/demos/aspnet/prometheus/Upload/Examples/UsingTargetFolder/DefaultCS.aspx
If the problem still persists, as requested in the ticket, we will need you to send us a sample app in your support ticket that reproduces this behaviour in Firefox.
Looking forward to hearing from you.
the Telerik team