Ciao.
Questo snippet funziona con IE
ma su FF non vede la variabile data:
Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript" type="text/JavaScript">
function DOMize(element) 
{
/////
/// DOMize function by andr3a 
////
    if(!element.addEventListener)
    {
        element.addEventListener=function(event,listener)
                                {
                                    event="on"+event;
                                    if(this.attachEvent)
                                    {    
                                        this.attachEvent(event,listener);
                                    }
                                    else 
                                    {            
                                        this[event]=listener;
                                    }
                                 }

    };
    if(!element.removeEventListener)
    {
        element.removeEventListener=function(event,listener)
                                    {
                                        event="on"+event;
                                        if(this.detachEvent)    
                                        {
                                            this.detachEvent(event,listener);
                                        }
                                        else     
                                        {        
                                                delete this[event];
                                        }
                                    }
    };
    return element;
};
function XmlHttpRequest() 
{
        var Request = null;
        try {
            Request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch(e) {
            try {
                Request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch(oc) {
                Request = null;
            }
        }
        if (!Request && typeof(XMLHttpRequest) != "undefined") 
        {
            Request = new XMLHttpRequest();
        }
        if (!Request) 
        {
            alert("Failed to create new ajax request.");
        }
        return Request;
};
var RequestSend = XmlHttpRequest();
function sendMessage(){
    var user="Pippo";
    var message = document.getElementById("userInput").value;
    if(message.length>100)
    {
            message=message.substring(0,100)
    }
       if (RequestSend.readyState == 4 || RequestSend.readyState == 0) 
    {
        var message = encodeURI(message);
        var user = encodeURI(user);
        var Content = "user="+user+"&message="+message;
        // open socket connection
        RequestSend.open("POST","sendchatdata.php",true);
        // set form http header
        RequestSend.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        RequestSend.setRequestHeader("Content-length", Content.length);
          RequestSend.setRequestHeader("Connection", "close");
        RequestSend.send(Content);
        RequestSend.onreadystatechange=senderStatusChecker;
     }
};
function senderStatusChecker()
{
     // if request is completed
    if(RequestSend.readyState==4)
    {
        // if status == 200 display text file
        if(RequestSend.status==200)
        {
            data=RequestSend.responseText;
            displayChatData();  
        }
        else
        {
            alert('Failed to get response :'+ receiveReq.statusText);
        }
    }
}
function displayChatData()
{

///////////////////////
// SU IE DATA E' OK SU FF NON LA VEDE                    
/////////////////////

alert("pippo");
alert(data);
            var chat = document.createElement("

");
            chat.setAttribute("id","chat");
            var chatOutPut = document.createTextNode(data);
            chat.appendChild(chatOutPut);
            if(check = document.getElementById("chat"))
            {
                document.getElementById("container").removeChild(check);
            }
            document.getElementById("container").appendChild(chat);
            check = document.getElementById("

");
}
window.onload = function()
{
    var send = document.getElementById("send");
    DOMize(send).addEventListener("click",sendMessage, false); 
}
</script>

</head>

<body>
<form action="" method="post" name="frm">
<input  id="userInput" name="userInput" type="text">
<input  id="send" name="send" type="button">
</form>
<div id="container"></div>
</body>
</html> 
Non riesco proprio a capire cosa c'è
di sbagliato scope (ma data è globale )
o cos'altro e
e ci aggiungo anche un grossisimo PP