Javascript Not working in Firefox but in IE works fine.

function GetData()
 {
  var objXML,bNewData,strUserID,strCoID,strAdmin;
  var strUserName,bNewData;
  var strSQL,objNode,xmlDoc;
  var xmlhttp;
  var objNode,objNodes;
  strUserID = Form1.txtUserID.value;
  strCoID= Form1.txtCoID.value;
  strAdmin= Form1.chkIsAdmin.checked;
  doClearTable(document.all.tblUsers);
  strSQL = 'url';
  bNewData = false;
  if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
   else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
   xmlhttp.open("GET",strSQL,false);
   xmlhttp.send();
 xmlDoc=xmlhttp.responseXML;
   objNodes=xmlhttp.evaluate(xmlDoc,xmlhttp,null,null,null);
   for(objNode=0; objNode<objNodes.length; objNode++)
   {
        strUserName=objNodes[objNode].selectSingleNode("UserName").text;
        strUserID=objNodes[objNode].selectSingleNode("ID").text;
        doAddSingleUserRow(strUserName,tblUsers,strUserID);
    }
 }

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