Loadvars values not loaded in Firefox form

Finding out my that my values are not loading the formfields in Firefox while they are loading perfect in IE i set up a small test.

The codes are fine with IE and give a problem with Firefox and Safari

 
//Sending values.
knop3.onRelease = function() {
 zend = new LoadVars();
 ontvang = new LoadVars();
 zend.testcode = testcode.text;
 zend.sendAndLoad("betalingff.php",ontvang,"POST");
 // terugmelden
ontvang.onLoad = function(success) {
 if (success)  {
  getURL("betalingff.php");
 } else {
  Fout.text = "Betalen met iDEAL is nu niet mogelijk, probeer het later nogmaals a.u.b.";
 }
 }
 
}

My PHP file

 
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
<?php
### bedrag moet geconverteerd worden naar eurocenten ###
$amount = $_POST['testcode'];
?>
De betaling afronden in de veilige omgeving van uw bank.
<FORM METHOD="post" ACTION="https://ideal.rabobank.nl/ideal/mpiPayInitRabo.do" id=form1 name=form1>
<INPUT type="hidden" NAME="amount" VALUE="<?php print $amount ?>" >
<INPUT name="Submit" type="submit" value="Betaal met iDEAL">
</form>
</body>
</html>
 

“<?php print $amount ?>” is correctly filled in IE and keeps blank in Firefox. Been cracking my head for over two weeks.
Appreciate some help.