Which FTP mode should be used by a client behind a firewall that has no special configurator?

In Passive mode, the FTP client sends a PASV command, and the server responds with an IP and port for the client to connect to for the data channel. Since all connections originate from the client side, stateful firewalls with no special FTP configuration allow the traffic without needing to permit inbound server-initiated connections.

Secure FTP (SFTP or FTPS) addresses encryption and authentication, not the firewall traversal problem caused by data channel directionality.

Active FTP has the server initiate the data connection back to the client on a high port, which is typically blocked by firewalls that deny unsolicited inbound sessions.

'Protected FTP' is not a recognized FTP operating mode defined in any standard.