Yes, I am beginning to suspect the problem lies in the node-red flow I am running on the raspberry pi. I never had the problem before when I ran firmata on the Arduino and node red on the pi, when I used node red flows using their Arduino nodes, which are designed to communicate with an Arduino running firmata. But I am now using their serial nodes, which are of course more generic, and now can't get it to connect to the simplest sketch, except by unplugging the usb plug and plugging it back in each time I reboot the pi.