From MozillaZine Knowledge Base
Ports are used when any program accesses the Internet so that the system can keep separate applications' data separate. Some port numbers are reserved for functions such as e-mail or FTP. To prevent potential security risks if a protocol was allowed access a port reserved for a seperate protocol, Gecko applications contain a list of banned ports. This preference allows you to augment the list of banned ports.
Possible values and their effects
A comma delimited list of port numbers to additionally block.
First checked in
Has an effect in