javascript中的Ajax函数在mozilla firefox浏览器中不起作用，为什么？

Question

Follownig是我的JavaScript代码，ajax函数用于读取响应     服务器。但是在Firefox浏览器中xmlhttp.status始终为0，为什么？请帮我。

        function ajaxAsyncRequest() {
            var xmlhttp;
            if (window.XMLHttpRequest) {
                xmlhttp = new XMLHttpRequest();
            } else {
                xmlhttp = new ActiveObject("Microsoft.XMLHTTP");
            }

            //creating asynchrounous GET request

            var tempValUrl = $(".urlVal").val();
            var urls = tempValUrl + '/Department/departmentAdminTokenReceive';
            xmlhttp.open("GET", urls, true);
            xmlhttp.send();

            xmlhttp.onreadystatechange = function() {

                if (xmlhttp.readyState == 4) {

                    alert("readyState"+xmlhttp.readyState);
                    alert("status"+xmlhttp.status);
                    if (xmlhttp.status == 200) {

                        var data = xmlhttp.responseText;
                        //for signing
                        var signarr = data.split("$$");
                        //for signing 
                        document.getElementById('signName').value = signarr[1];
                        document.getElementById('signCertName').value = signarr[2];
                        document.getElementById('signCa').value = signarr[3];   
                        document.getElementById('signExpiryDate').value = signarr[4];
                        document.getElementById('signPublicKey').value = signarr[5];
                    }
                }
            }
        }