Squid normally uses FTP’s so-called passive mode for file transfers. This means that the FTP server creates a TCP socket for data transfer and waits for the client to connect. Passive mode works much better through most Internet firewalls. The alternative is to have the FTP client (Squid in this case) create a TCP socket and wait for a connection from the server. Most likely, you’ll never have problems with FTP passive mode. However, you can force nonpassive operation by turning off this directive.
ftp_user, ftp_list_width, ftp_sanitycheck