Cambiando motore la cosa
non cambia:

Codice PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script language="JavaScript" type="text/JavaScript">
 var http_request = false;
   function makePOSTRequest(url, parameters) {
      http_request = false;
      if (window.XMLHttpRequest) { // Mozilla, Safari,...
         http_request = new XMLHttpRequest();
         if (http_request.overrideMimeType) {
             // set type accordingly to anticipated content type
            //http_request.overrideMimeType('text/xml');
            http_request.overrideMimeType('text/html');

         }
      } else if (window.ActiveXObject) { // IE
         try {
            http_request = new ActiveXObject("Msxml2.XMLHTTP");
         } catch (e) {
            try {
               http_request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {}
         }
      }
      if (!http_request) {
         alert('Cannot create XMLHTTP instance');
         return false;
      }
      
      http_request.onreadystatechange = alertContents;
      http_request.open('POST', url, true);
      http_request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
      http_request.setRequestHeader("Content-length", parameters.length);
      http_request.setRequestHeader("Connection", "close");
      http_request.send(parameters);
   }

   function alertContents() {
         document.getElementById('myspan').innerHTML ='loading ......';
      if (http_request.readyState == 4) {
         if (http_request.status == 200) {
            //alert(http_request.responseText);
            result = http_request.responseText;
            document.getElementById('myspan').innerHTML = result;            
         } else {
            alert('There was a problem with the request.');
         }
      }
   }
window.onload= function(){ 
    var target = document.getElementById('my-buttom'); 
    if(!target){return;} 
    target.onclick=function(){ 
    var inputValue = document.getElementById('my-text').value; 
    var postVar= 'cat_title='+encodeURIComponent(inputValue); 
    //alert(postVar); 
     makePOSTRequest('post.php', postVar);
    } 
}
</script>

</head>

<body>
<form action="" method="post" name="frm"> 
<input id="my-text" name="my_test" type="text">
 
<input id="my-buttom" name="my_buttom" type="button" value="insert">
</form>



Server-Response:

<hr>
<span name="myspan" id="myspan"></span>
<hr>
</body>
</html> 
Firebug da watch out lo script mioinput ?

Vi risulta questo comportamento o no ?

Comincio a pensare che ho Firebug bacato

riportao le versioni

FF 2.0.0.5

Firebug 1.05