[Flickr APIs] xmlhttprequest help

Hi,

I am a happy flickr user and hobbyist enthusiast developer, a newbie
starting to play with Flickr API. I am using this XML HTTPRequest
tutorial as a starting point:
http://developer.apple.com/internet/webcontent/xmlhttpreq.html

I have made a simple javascript test app that calls
flickr.photosets.getPhotos to generate an html code with all
thumbnails from a set. The code is below:

-----------------
var url = 
"http://www.flickr.com/services/rest/?method=flickr.photosets.getPhotos";
var user_id = "49503114626@N01"
var photoset_id = "3913"
var api_key = "72fadaff0fad605613c6caeb8a09f85d"
var params = "&api_key="+api_key+"&photoset_id="+photoset_id
var req;

function loadXMLDoc(url) {
    // branch for native XMLHttpRequest object
    if (window.XMLHttpRequest) {
        req = new XMLHttpRequest();
        req.onreadystatechange = processReqChange;
        
netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");
        req.open("GET", url, true);
        req.send(null);
    // branch for IE/Windows ActiveX version
    } else if (window.ActiveXObject) {
        req = new ActiveXObject("Microsoft.XMLHTTP");
        if (req) {
            req.onreadystatechange = processReqChange;
            req.open("GET", url, true);
            req.send();
        }
    }
}

function processReqChange() {
    // only if req shows "loaded"
    if (req.readyState == 4) {
        // only if "OK"
        if (req.status == 200) {
                        var xmlResponse = req.responseXML;
                        if(xmlResponse.documentElement){
                                var idphotos = []
                                var photoset = 
xmlResponse.documentElement.firstChild //photoset
                                for(var photo = 
photoset.firstChild;photo!=null; photo = photo.nextSibling){
                                        idphotos.push(photo.getAttribute("id"))
                                }
                                printPhotos(idphotos)
                        }else{
                                alert("error")
                        }
        } else {
            alert("There was a problem retrieving the XML data:\n" +
                req.statusText);
        }
    }
}
function printPhotos(ids){
        var str = ""
        for (var i=0; i<ids.length;i++){
                str +="<a 
href='http://www.flickr.com/photo.gne?id="+ids[i]+";'><img
src='http://www.flickr.com/photos/"+ids[i]+"_"+user_id+"_s.jpg'
/></a>"
        }
        document.write(str)
}
loadXMLDoc(url+params)
-----------------

It works fine if I run it locally on IE, but on Firefox I receive the
follow error:

Error: [Exception... "'Permission denied to get property
XMLDocument.documentElement' when calling method:
[nsIOnReadystatechangeHandler::handleEvent]"  nsresult: "0x8057001e
(NS_ERROR_XPC_JS_THREW_STRING)"  location: "<unknown>"  data: no]


Anyone knows what could be wrong, or some other way to make it?

PS: I dont have one API Key yet, I had requested one by email but
meanwhile I am using a borrowed one from the flickr demo pages :(

-- 
Fabricio C Zuardi
http://idomyownstunts.blogspot.com