I am not sure about the browser version that you are using. Since you are using older DLL version it doesn't have full support on the latest browser. If you would like to ensure that AJAX controls are compatible with
the new major versions of IE 9, FireFox 4 and Google Chrome10 browsers, you will need to migrate
at least to version 2011.1.315.Also Telerik supports Firefox 5 officially with the 2011 R1 Service Pack 2 . With older versions these browsers are
only partially supported.