[Javascript] Firefox et Opera VS Safari et IE

function getstatus() {

    var xhr = null;

    if(window.XMLHttpRequest) // Firefox
    xhr = new XMLHttpRequest();
    else if(window.ActiveXObject) // Internet Explorer
    xhr = new ActiveXObject("Microsoft.XMLHTTP");
    else { // XMLHttpRequest non supporté par le navigateur
    alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest...");
    return;
}

var idProgress = document.getElementById("clef").value;

xhr.open("GET", "progress.php?progress_key="+idProgress, true);
xhr.onreadystatechange = function() {
    if(xhr.readyState == 4) {
        if(xhr.status == 200) {

        var tmp=xhr.responseText;
        var reponse=eval('('+tmp+')');
       
        var b_up = reponse.bytes_uploaded;
        var b_tot = reponse.bytes_total;
        var percent = Math.ceil(b_up*100/b_tot);
        var pixel = percent*300/100;
       
        document.getElementById("sec").innerHTML = reponse.est_sec+" secondes restantes";
        document.getElementById("p_pc").innerHTML = percent+"%";
        document.getElementById("p_barre").style.width=parseInt(pixel)+"px";
        document.getElementById('rep').style.display = '';
        if (percent!=100) { getstatus(); }
        }
    }
}

xhr.send(null);

}