A possible workaround:
<rad:RadEditor ID="RadEditor1" runat="server" OnClientCommandExecuted="OnClientCommandExecuted"></rad:RadEditor>
function OnClientCommandExecuted(editor, args) {
                if ("Unlink" == args.get_commandName()) {
                    if ($telerik.isFirefox) {
                        var anchor = editor.getSelectedElement();
                        if (anchor && anchor.nodeName == "A") {
                            Telerik.Web.UI.Editor.Utils.removeNode(anchor);
                        }
                    }
                }
            }