From MozillaZine Knowledge Base

All URIs and URLs start with a specific scheme such as HTTP or FTP. Mozilla uses XPCOM components to implement protocol handlers for each type of scheme it supports. These components can be written in C/C++ or JavaScript.

Documentation

• Adding a New Protocol to Mozilla
• Mozilla.org - Writing A Mozilla Protocol Handler
• Mozilla.org - URIs and URLs
• LXR page for nsIProtocolHandler interface

Protocol Handlers

• Mozilla.org - Finger Protocol Handler
• Extend the Chrome Protocol Handler