Submit form i Ajax

Jeg har en simpel form som submitter på siden uden reload via ajax.
Det hele virker fint. Det vil sige, det trode jeg. Det kører fint i IE, men når jeg prøver i Firefox sker der intet når jeg trykker submit. Er der nogle som kan se hvad der skal gøres anderledes for at det skal virke i begge browsere?


<form action="java script:get(document.getElementById('myform'));" name="myform" id="myform" method="POST">

<script type="text/javascript" language="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() {
      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.');
        }
      }
  }
 
  function get(obj) {
      var poststr = "comment=" + encodeURI( document.getElementById("comment").value );
    poststr = poststr + "&id=" + encodeURI( document.getElementById("id").value );
      makePOSTRequest('control.asp?postcomment=yes', poststr);
  }
</script>