Overview of Mozilla embedding APIs
contract-id: ns_uri_loader_contractid implemented interfaces: nsiuriloader related interfaces: nsiuricontentlistener nsunknowncontenttypehandler the unknowncontenttypehandler service is the last resort of the uriloader when no other content handler can be located.
...interface definition: nsiuriloader interface status...

Document Loading - From Load Start to Finding a Handler - Archive of obsolete content
nsuriloader this is a service implementing nsiuriloader.

nsIExternalProtocolService
this parameter may be ultimately passed nsiuriloader.openuri() in the case of a web handler, and awindowcontext is null or not present, web handlers will fail.