Questo dovrebbe funzionare sia per IE che per Firefox.
	codice:
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function addRow(aTable) {
 newRow      = aTable.insertRow(-1);
 //----------- Cella 1
 newCell             = newRow.insertCell(-1);
 newCell.className   = "calBtn";
 newCell.innerHTML   = '<input type=button class=calBtn value="<<" onclick="changeCal(previousYear,currentMonth+1)">';
 newCell.align       = "center";	 
 newCell.vAlign      = "top"; 
 //----------- Cella 2
 newCell             = newRow.insertCell(-1);
 newCell.className   = "calBtn";
 newCell.innerHTML   = '<input type=button class=calBtn value="<" onclick="changeCal(currentYear,previousMonth+1)">';
 newCell.align       = "center";	 
 newCell.vAlign      = "top";  
 //----------- Cella 1
 newCell             = newRow.insertCell(-1);
 newCell.className   = "calBtn";
 newCell.innerHTML   = '<input type=button class=calBtn value="<<" onclick="changeCal(previousYear,currentMonth+1)">';
 newCell.align       = "center";	 
 newCell.vAlign      = "top";  
}
//-->
</script>
</head>
<body>
<table id="tabella" border="1">
<tr>
<td>AAAAA</td><td>BBBBB</td><td>CCCCC</td>
</tr>
</table>
<input type="button" value="Inserisci" onclick="addRow(document.getElementById('tabella'))";
</body>
</html>