Both Mozilla and Netscape support the irc: and mailto: protocol identifiers.
Actually, it's not even an issue of support... just like any URL/pathname aware program should, anything that's not nativly handled is simply passed off to the OS to decide what to do with it. So if you add the foo: identifier to the system, and type <a href="foo:///bar" target="_blank">foo:///bar</a> in the location field it'll launch what ever handler it's configured for.
Same as with system registered MIME types.
