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. On Linux and Mac systems, this preference acts as an override to the protocol-specific network.protocol-handler.expose.(protocol) preferences.
Possible values and their effects
All protocols should be handled. (Default)
Protocols should be handled based on their specific network.protocol-handler.expose.(protocol) value.
First checked in
Has an effect in