Grazie dei consigli. Allora ho copiato il codice html del file erbario.php restituitomi da Firefox.

Eccolo:

codice:
<html><head>
<meta http-equiv="content-type" content="text/html; charset=ISO-8859-1"> 
	<meta name="author" content="Ignazio Sammarco">
	<link href="erb_files/erbario.css" rel="stylesheet" type="text/css">
	<script src="erb_files/prototype.js" type="text/javascript"></script>
	<script src="erb_files/effects.js" type="text/javascript"></script>
	<script src="erb_files/scriptaculous.js" type="text/javascript"></script><script type="text/javascript" src="erb_files/builder.js"></script><script type="text/javascript" src="erb_files/effects.js"></script><script type="text/javascript" src="erb_files/dragdrop.js"></script><script type="text/javascript" src="erb_files/controls.js"></script><script type="text/javascript" src="erb_files/slider.js"></script><script type="text/javascript" src="erb_files/sound.js"></script>

        <link rel="stylesheet" href="erb_files/style.html">



<title>Universita' degli Studi di Palermo: Erbario micologico</title>

<script type="text/javascript">

// Funzione per mostrare tutti i ritrovamenti del fungo con id = str(passata come parametro), nella tabella erb_erbario
function showFunghiErbario(str, str2)
{
  var xmlhttp;
  if (str=="")
  {
    document.getElementById("txtHint").innerHTML="";
    return;
  }
  if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
  }
  else
  {// code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
  xmlhttp.onreadystatechange=function()
  {
    if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
      document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
    }
  }
  xmlhttp.open("GET","getfunghierb.php?idfungo="+str,true);
  
  xmlhttp.send();
}

</script></head>






<body onload="document.frm.reset();"> 

<script type="text/javascript">

window.onload = function()
{
document.getElementById('specie').focus()
}

</script> 
 

<form name="frm" action="/erbario/erbario.php" method="POST">					
	<input name="inv_idfungo" value="" type="hidden">
	<input name="inv_idlocalita" value="" type="hidden">
	<input name="inv_idhabitat" value="" type="hidden">
	
	<div style="position:absolute; left:620px; top: 0px; width:200; padding: 1em;"> 
	<h2>Ultima Scheda: 5145 </h2> 

	</div>

    <div style="position:absolute; top: 0px; width:160px; padding: 1em;">
        <h2>Specie:</h2>
        <input autocomplete="off" id="specie" name="specie" type="text"> 
		        

        

        <div id="txtHint" class="txtLocalita"> </div>
    </div>

    <div style="display: none;" id="suggerimenti_specie" class="boxsuggerimenti"></div>
    <script type="text/javascript">
	new Ajax.Autocompleter("specie", "suggerimenti_specie", "cerca.php", {minChars: 2, afterUpdateElement: getSelectionId});
	function getSelectionId(text, li){
		showFunghiErbario(li.id);
		document.getElementById('inv_idfungo').value=li.id;
	}
    </script>
	<div id="aggErb" style="position:absolute;left:620px; top: 50px; width:200px; padding: 1em; background-color:#afec80;"> 
		Localita': <input name="localita" type="text"> 
		        

	<div id="suggerimenti_localita" class="boxsuggerimenti"></div>
		<script type="text/javascript">
			new Ajax.Autocompleter("localita", "suggerimenti_localita", "cerca.php", {minChars: 2, afterUpdateElement: getSelectionId});
			function getSelectionId(text, li){
			document.getElementById('inv_idlocalita').value=li.id;
			}	
		</script>
	Comune: <input disabled="disabled" name="comune" type="text"> 
	

	Provincia: <input disabled="disabled" name="provincia" type="text"> 

	

	Data ritrov.: <input name="data_ritr" type="text"> 

	Habitat: <input name="habitat" <br="" type="text">
	<div id="suggerimenti_habitat" class="boxsuggerimenti"></div>
	<script type="text/javascript"> 
	  new Ajax.Autocompleter("habitat", "suggerimenti_habitat", "cerca.php", {minChars: 2, afterUpdateElement: getSelectionId});
	  function getSelectionId(text, li){
		document.getElementById('inv_idhabitat').value=li.id;
}	</script>
    

    

	Substrato: <input name="substrato" type="text"> 

        Leg.: <input name="leg" type="text">         

        Det.: <input name="det" type="text"> 

	Note: <textarea name="note" cols="40" rows="2"></textarea> 

	

	<input name="action" value="aggiungi" type="submit">
   </div>
</form>
 
 </body></html>

Il file css è lo stesso di prima.