No first_name or user_firstname property in WP_User object

You have to use the function get_user_meta

$new_user = get_userdata($user_id);

// get all the meta data of the user
$new_user_data = get_user_meta( $user_id );

// get the first name of the user as a string
$new_user_firstname = get_user_meta( $user_id, 'first_name', true );