ajax IE下正常，FF下单步执行正常，连续执行不正常。

JScript code:
function GetXmlHttpObject()
{
try
{
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}
catch (e)
{
// Internet Explorer
try
{
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e)
{
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
}
return xmlHttp;
}

JScript code:
function btnOk_Click()
{
var username = document.getElementById("txtName").value;
var password = document.getElementById("txtPass").value;
var xmlHttp = GetXmlHttpObject();
if(xmlHttp!=null)
{
xmlHttp.onreadystatechange=function()
{
if(xmlHttp.readyState == 4)
{
if(xmlHttp.responseText == "1") // login successfully
{
document.getElementById("div1").style.display="none";
document.getElementById("div2").style.display="block";
return true;
}
else if(xmlHttp.responseText == "0") // username or password is wrong
{