is based on RadEditor Classic version 7.3.6. This version of the classic editor is released on 3/6/2009 and it does not support IE8, IE9, Firefox 4+.
The development and support for RadEditor Classic is discontinued since 2009. You can find information about Telerik's Support Lifecycle Policy in this article: Telerik's Support Lifecycle Policy
as well as see the following chart: http://www.telerik.com/libraries/ajax_product_pages/radcontrols-lifecycle.sflb
the Telerik team