Dual Selecting Combo Boxes

function findTestTypes(grp){

  var url = "/tests/lookup?cmd=test&gc="+grp.value;
 
 if (window.XMLHttpRequest) {
        req = new XMLHttpRequest();
    } else if (window.ActiveXObject) {
        req = new ActiveXObject("Microsoft.XMLHTTP");
 }
 
 req.open("GET", url, true);
 req.onreadystatechange = showTestTypes;
 req.send(null);  
}

function showTestTypes(){
 if (req.readyState == 4 && req.status == 200) {
     var html = req.responseText;
     var tt = getItem("tests");
     tt.innerHTML = html;

     fireChangeEvent("test_code");
    }
}

// for mozilla
function fireChangeEvent(fieldName){
     if (window.ActiveXObject) {
      getItem(fieldName).fireEvent('onchange');
     }else{
     // target is some DOM node on which you wish to fire an event.
   var target = getItem(fieldName);
   
   var oEvent = document.createEvent( "Events" );
   oEvent.initEvent(
     "change",    // the type of mouse event
     true,       // do you want the event to
                 // bubble up through the tree?  (sure)
     true,       // can the default action for this
                 // event, on this element, be cancelled? (yep)
     window,     // the 'AbstractView' for this event,
                 // which I took to mean the thing sourcing
                 // the mouse input.  Either way, this is
                 // the only value I passed that would work
     1,          // details -- for 'click' type events, this
                 // contains the number of clicks. (single click here)
     1,          // screenXArg - I just stuck 1 in cos I
                 // really didn't care
     1,          // screenYArg - ditto
     1,          // clientXArg - ditto
     1,          // clientYArg - ditto
     false,      // is ctrl key depressed?
     false,      // is alt key depressed?
     false,      // is shift key depressed?
     false,      // is meta key depressed?
     0,          // which button is involved?
                 // I believe that 0 = left, 1 = right,
                 // 2 = middle
     target       // the originator of the event
                 // if you wanted to simulate a child
                 // element firing the event you'd put
                 // its handle here, and call this method
                 // on the parent catcher.  In this case,
                 // they are one and the same.
   );
   target.dispatchEvent( oEvent );     
      
     }
}