another Firefox problem

[blckspder]

I have been looking at other posts about Firefox and AJAX. I haven't found anything that will solve my problem.... not anything I've noticed anyways. My code works perfectly in IE but not Firefox. I am using version 2.0

here is my code

<script language="javascript" type="text/javascript">
var XMLHttpRequestObject = false;
try{
    XMLHttpRequestObject = new ActiveXObject("MSXML2.XMLHTTP");
    } catch(exception1) {
        try{
            XMLHttpRequestObject = new ActiveXObject("Microsoft");
            } catch(exception2) {
                XMLHttpRequestObject = false;
                }
        } 

if(XMLHttpRequestObject && window.XMLHttpRequest) {
    XMLHttpRequestObject = new XMLHttpRequest();
    }
    
function getValue(cat,id) {
    clearList(cat,id);
    getSub(cat,id);
    } 
    
function getSub(cat,id) {
    if(XMLHttpRequestObject) {
        if(cat == "Electronics") {
            XMLHttpRequestObject.open("GET","electronics.xml",true);
            }
        if(cat == "Medical") {
            XMLHttpRequestObject.open("GET","medical.xml",true);
            }
        if(cat == "Industrial") {
            XMLHttpRequestObject.open("GET","industrial.xml",true);
            }
        if(cat == "Nanotechnology") {
            XMLHttpRequestObject.open("GET","nano.xml",true);
            }
        alert(cat);
        var subId = id;
        
        XMLHttpRequestObject.onreadystatechange = function() {
            if(XMLHttpRequestObject.readyState == 4 && XMLHttpRequestObject.status == 200) {
                var xmlDocument = XMLHttpRequestObject.responseXML;
                
                var subCategory = xmlDocument.getElementsByTagName("subs");
                listSubs(subCategory,subId);
                }
            }
        XMLHttpRequestObject.send(null);
        }
    }
    
function listSubs(subCategory,subId) {
    var loopIndex;
    var subCat;
    if(subId == "mainCat1") {
        subCat = "subCat1";
        }
    if(subId == "mainCat2") {
        subCat = "subCat2";
        }
    var selectControl = document.getElementById(subCat);
    
    for(loopIndex=0;loopIndex<subCategory.length;loopIndex++) {
        selectControl.options[loopIndex] = new
            Option(subCategory[loopIndex].firstChild.data);
        }
    }
    
function clearList(cat,id) {
    var clearCat;
    if(id == "mainCat1") {
        clearCat = "subCat1";
        }
    if(id == "mainCat2") {
        clearCat = "subCat2";
        }
    var len = document.getElementById(clearCat).options.length
      for(i = 0; i < len; i++) {
        document.getElementById(clearCat).options[0] = null;
        }
    }
</script> 

I hope someone can spot something. Thanks for your help.