WooCommerce: Add First Name to Order Email Subject

/**
 * @snippet       Add Billing First_name to Email Receipt (Customer Processing)
 * @how-to        Get tutoraspire.com FREE
 * @author        Tutor Aspire
 * @testedwith    WooCommerce 3.8
 * @donate $9     https://www.tutoraspire.com
 */
 
add_filter( 'woocommerce_email_subject_customer_processing_order', 'tutoraspire_change_processing_email_subject', 10, 2 );
 
function tutoraspire_change_processing_email_subject( $subject, $order ) {
   $subject = $order->get_billing_first_name() . ', thanks for your order!';
   return $subject;
}