Ho modificato una porzione del tuo codice, vedi se funziona:

<?php
$conture = 0;
do {
$counter++;
?>
<table width="87%" height="25" border="0">
<tr>
<td width="17%" height="21"> <div align="center"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><?php echo $row_Recordset['TIPO_CARTONE']; ?></font></div></td>
<td width="32%"><div align="center"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><?php echo $row_Recordset['MIS_ESTERNA']; ?></font></div></td>
<td width="13%"><div align="center"><font size="2" face="Verdana, Arial, Helvetica, sans-serif">
<input name="QUANTITA" type="text" id="QUANTITA<?php echo $counter; ?>" value="<?php echo $row_Recordset['QUANTITA']; ?>" size="10">
</font></div></td>
<td width="13%"> <div align="center">
<input name="PREZZO" type="text" id="PREZZO<?php echo $counter; ?>" size="10">
</div></td>
<td width="12%"><div align="center">
<input name="SCONTO" type="text" id="SCONTO<?php echo $counter; ?>" size="10">
</div></td>
<td width="13%"><div align="center">
<input name="IMPORTO" type="text" id="IMPORTO<?php echo $counter; ?>" onFocus="importo(this, <?php echo $counter; ?>)" value="" size="10">
</div></td>
</tr>
</table>
<?php } while ($row_Recordset = mysql_fetch_assoc($Recordset)); ?>