Thank you for getting back to me and reporting that the issue also exists in Firefox 4.
I understand your frustration with the inability of RadComboBox to filter Turkish characters properly. Generally it should work with any character set flawlessly.
Since RCB is on the queue to undergo heavy bug fixing during this Q, we will also consider improving its filtering functionality to support Turkish as well as other characters.
The PITS item has been updated with Firefox 4 as well.
If you have additional questions, please let us know.
the Telerik team