dropdownbox not working in php and ajax

suthagar

Hi friends,
I created simple drop down box in php and ajax that second dropdownbox is depend by first dropdownbox,this is perfectly working in IE and opera but not working in mozzila, i just give the code below please help me to work in mozzila browser also,

// my index page - index.php

<html>
<head>
<script src="responsexml.js"></script>

</head>
<body>

<form>
Select a Country:
<select name="users" onChange="showUser()"> // first dropdown & calling js on onchange
<option value="0">Select Country</option>
<option value="1">INDIA</option>
<option value="2">USA</option>
<option value="3">CANADA</option>
</select>
<select name="DropDownList1" id="DropDownList1"> //secong dropdown
</select>
</form>
</body>
</html>

// my javascrit file - responsexml.js

var xmlHttp;
function GetXmlHttpObject()
{
var objXMLHttp=null;
if (window.XMLHttpRequest)
{
objXMLHttp=new XMLHttpRequest();
}
else if (window.ActiveXObject)
{
objXMLHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
return objXMLHttp;
}

function showUser()
{
xmlHttp=GetXmlHttpObject();
if (xmlHttp==null)
{
alert ("Browser does not support HTTP Request");
return;
}

 var url="responsexml.php";     
 var str1=document.getElementById("users").value;
 url=url+"?q="+str1;
 url=url+"&sid="+Math.random();
 xmlHttp.open("GET",url,true);
 xmlHttp.onreadystatechange=AssignCity;
 xmlHttp.send(null);

}