From MozillaZine Knowledge Base
The FTP specification dictates that users must give a username and a password when logging in to a server. If a server allows anonymous logins, clients usually log in with the username “anonymous” and a password corresponding to an email address. If the preference advanced.mailftp is set to true, Mozilla will send the value of this preference instead.
Possible values and their effects
This string is sent in the PASSWORD response to the FTP server immediately following the server’s 331 response to the USER anonymous message. Traditionally the password for anonymous accounts is an email address (and some servers actually check to see if the password looks like one), but you may specify anything you like.
A text field is located beneath a checkbox labeled “Send this email address as anonymous FTP password:” under “Edit → Preferences → Advanced”.
First checked in
Has an effect in