SO fiddle

Test Fiddle mainly for SO Questions

by Nick Craver

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/overcast/jquery-ui.css">
<table>
<tr>
    <td><font size="1"><b>0</b></font></td>
    <td><font size="2"><b>0</b></font></td>
    <td><font size="3"><b>3947</b></font></td>
    <td><font size="4"><b>0</b></font></td>
</tr>
</table>

JavaScript

$(document).ready(function(){
  var invoice_number = $("td font[size='3'] b").text();
  alert(invoice_number);
});