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 controls whether to warn the user before opening an external application for a specific protocol.
Possible values and their effects
Give the user a chance to cancel before passing the data to the external application.
Pass the data to the external application without warning the user.
On the protocol warning dialog, "Remember my choice for all links of this type"
First checked in
Has an effect in