Hi, if the above code don't work for you
Just try no setting name to the window popup
echo '<a href="#" onclick="window.open(\'mail_compose.php?pid=' . $pop . '\', \'\', \'width=900, height=600, left=100, top=100\'); return false;">Click here</a>';
With this, the browser open as many windows as you want
Itested it on firefox 3.6.8 and IE 8