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 determines whether a specific protocol should be handled at all (either internally or externally).
Possible values and their effects
This protocol should be handled either by the browser or by an external application.
This protocol should not be handled by the browser, nor by an external application.
First checked in
Has an effect in