初学JSON，创建Ajax请求，火狐控制端报“Ajax is not defined”,该如何处理

<script type="text/javascript" src="js/json.js"></script><script type="text/javascript" src="prototype.js"></script><script type="text/javascript">            function sendRequest(){                var url="/JSON/JSONTest1";                alert("json");                var mailAjax=new Ajax.Request(                            url,                            {                                method:'get',                                onComplete:jsonResponse                            }                        );                alert("json");            }            function jsonResponse(originalRequest){                alert(originalRequest.responseText);                var myobj=originalRequest.responseText.parseJSON();                alert(myobj.name);            }</script>

PrintWriter out=resp.getWriter();out.print("{\"name\":\"liuyong\",\"age\":21}");out.flush();out.close();