From MozillaZine Knowledge Base
Protocols (ex: http, ftp, mailto) are different methods of sending information over the Internet. Protocols may be handled by Mozilla applications themselves, or they may be passed to external applications, or they may not be handled at all. This preference determines whether a specific protocol that is already configured to be handled but whose behavior is not defined by a specific network.protocol-handler.external.(protocol) preference should be handled internally or externally.
Possible values and their effects
This protocol should be handled by an external application. (Default)
This protocol should be handled internally.
First checked in
Has an effect in