code: protected function setIsFirst(OrderInterface $order)
      {
          if (null !== $customer = $order->getCustomer()) {
              if ($customer->hasParent()) {
                  $customer = $customer->getParent();
              }
              $first = !$this->orderRepository->existsForCustomer($customer);
          } else {
              $first = !$this->orderRepository->existsForEmail($order->getEmail());
          }
  
          if ($first != $order->isFirst()) {
              $order->setFirst($first);
  
              return true;
          }
  
          return false;
      }
docs: Sets whether this order is the customer's first one.
  
  @param OrderInterface $order
  
  @return bool Whether the order has been changed.