I was curious about the firewall. Specifically, what would happen if I block access to port 8000 where Parsoid is listening to. Let's see.
# Allow SSH
sudo ufw allow ssh
# Allow HTTP
sudo ufw allow http
# Enable UFW
sudo ufw enable
# Check status
sudo ufw status
Nope. Does not work. I received this error when I was trying to edit the main page.
Error loading data from server: apierror-visualeditor-docserver-http-error: (curl error: 28) Timeout was reached. Would you like to retry?
Adding 8000 fixed it.