reading and parsing remote xml-file in Mozilla

I have these functions:

===================================
function LoadXMLFile(sURL) {
    var oXML;
    try {
        oXML = new ActiveXObject('Msxml2.DOMDocument');
    } catch(e) {oXML = false;}
    if(oXML) {
        //IE load XML document
        oXML.async = false;
        oXML.load(sURL);
        if(oXML.parseError.errorCode) {
            alert('error: ' + oXML.parseError.description);
            return false;
        }
    }
    if(!oXML) {
        //Mozilla load XML document
        try {
            oXML = document.implementation.createDocument("", "", null);
        } catch(e) {oXML = false;}
        if(oXML) {
            try {
                oXML.load(sURL);
            } catch(e) {oXML = false;}
        }
    }
    return oXML;
}



function ConfirmTask(iTaskId, iPersonId, oCheckbox) {
    var bChecked = oCheckbox.checked;
    var sURL = '../rooster/ConfirmPersonPlanning.asp?task_id='+iTaskId+'&person_id='+iPersonId+'&confirm='+bChecked;

    var oXML = LoadXMLFile(sURL);
    if(oXML) {
        var sResult = parseInt(oXML.firstChild.text);  // <<====== here is the line with the error
        alert(sResult);
        if(!sResult) {
            oCheckbox.checked = !oCheckbox.checked;
            alert('it did not work');
        } else {
            alert('ok');
        }
    } else {
        oCheckbox.checked = !oCheckbox.checked;
        alert('no support for this');
    }
    return false;
}
=======================

It works fine in IE, but Firebird (0.8)  returns an error

oXML.firstChild.text has no properties. Apparently something goes wrong the loading the XML. I looked at a lot of documentation, but i could not find a solution.

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