Objeto Ajax y Firefox??

[mount]

Suelo usar un código similar al que sigue, y no me ha dado problemas en Firefox:

Código:

    //create the ajax coonnection
    //for IE
    var req = null;
    if(window.ActiveXObject)
    {
          req = new ActiveXObject(”Microsoft.XMLHTTP”);
    }
    //for Firefox
    else if(window.XMLHttpRequest)
    {
          req = new XMLHttpRequest();
    }
    //define the file requested
    var url = document.form1.country.options[selected].value+”.xml”;
    //define which function to deal with the result of the request
    req.onreadystatechange = getCities;
    //using Post or get method to send the request,we use get method here
    req.open(”GET”,url, true);
    //send request to the server
    //there is a trick, when you don’t want to send any parameters
    //you need to use null, otherwise, there will be some problem in Firefox
       req.send(null);