Поправил код и все заработало как надо:
<?php
$db_host = 'localhost';
$db_name = 'bill';
$db_username = 'root';
#$db_password = 'admin09876';
$connect_to_db = mysql_connect($db_host, $db_username, $db_password)
or die("Could not connect: " . mysql_error());
mysql_select_db($db_name, $connect_to_db)
or die("Could not select DB: " . mysql_error());
$qr_result = mysql_query('SELECT * FROM users, invoices WHERE email="'.mysql_real_escape_string($_SESSION['email']).'"')
or die(mysql_error());
$data = mysql_fetch_array($qr_result);
echo '<tr>';
echo '<li>№' . $data['id'] .'</li>';
echo '<li>' . $data['first_name'] . ' ', $data['last_name'] . '</li>';
echo '<li>' .$data['number'].'</li>';
echo '<li>' . $data['email'] .'</li>';
echo '<li>' . $data['balance'] .'$</li>';
echo '<li>' . $data['service'] .'</li>';
echo '</tr>';
echo '<tr>';
echo '<li>№' .$data['invoice_id'].'</li>';
echo '<li>' . $data['invoice_ammount'] .'$</li>';
echo '</tr>';
while($data = mysql_fetch_array($qr_result)){
}
mysql_close($connect_to_db);
?>