nsIExternalProtocolService Interface

 Check whether a handler for a specific protocol exists.
 @param aProtocolScheme The scheme from a url: http, ftp, mailto, etc.
 @return true if we have a handler and false otherwise.

boolean externalProtocolHandlerExists(in string aProtocolScheme)

 Gets a human-readable description for the application responsible for
 handling a specific protocol.

 @param aScheme The scheme to look up. For example, "mms".

 @throw NS_ERROR_NOT_IMPLEMENTED
        If getting descriptions for protocol helpers is not supported
 @throw NS_ERROR_NOT_AVAILABLE
        If no protocol helper exists for this scheme, or if it is not
        possible to get a description for it.

AString getApplicationDescription(in AUTF8String aScheme)

 Check whether a handler for a specific protocol is "exposed" as a visible
 feature of the current application.

 An exposed protocol handler is one that can be used in all contexts.  A
 non-exposed protocol handler is one that can only be used internally by the
 application.  For example, a non-exposed protocol would not be loaded by the
 application in response to a link click or a X-remote openURL command.
 Instead, it would be deferred to the system's external protocol handler.

boolean isExposedProtocol(in string aProtocolScheme)

 Used to load a URI via an external application. Might prompt the user for
 permission to load the external application. Replaces loadUrl()

 @param aURI    The URI to load
 @param aPrompt If null we grab one from windowwatcher if we need it

void loadURI(in nsIURI aURI, in nsIPrompt aPrompt)

 Used to load a url via an external protocol handler (if one exists)
 @param aURL The url to load

void loadUrl(in nsIURI aURL)