nose si tendra algo que ver, pero si pruebas a cambiar:
esto:
Código PHP:
$sql = "select title, firstname, surname, company, address, postcode, tel, mobile_phone, fax, email, country from customers where (firstname = '$firstname')";
por esto:
Código PHP:
$sql = "select title, firstname, surname, company, address, postcode, tel, mobile_phone, fax, email, country from customers where (firstname = '".$firstname."') ";