nsIHandlerService in Firefox 3.0

 Retrieve a list of all handlers in the datastore.  This list is not
 guaranteed to be in any particular order, and callers should not assume
 it will remain in the same order in the future.

 @returns a list of all handlers in the datastore

 Whether or not a record for the given handler info object exists
 in the datastore. If the datastore is corrupt (or some other error
 is caught in the implementation), false will be returned.

 @param aHandlerInfo  a handler info object

 @returns whether or not a record exists

 Fill a handler info object with information from the datastore.

 Note: because of the way the external helper app service currently mixes
 OS and user handler info in the same handler info object, this method
 takes an existing handler info object (probably retrieved from the OS)
 and "fills it in" with information from the datastore, overriding any
 existing properties on the object with properties from the datastore.

 Ultimately, however, we're going to separate OS and user handler info
 into separate objects, at which point this method should be renamed to
 something like "get" or "retrieve", take a class and type (or perhaps
 a type whose class can be determined by querying the type, for example
 an nsIContentType which is also an nsIMIMEType or an nsIProtocolScheme),
 and return a handler info object representing only the user info.

 Note: if you specify an override type, then the service will fill in
 the handler info object with information about that type instead of
 the type specified by the object's nsIHandlerInfo::type attribute.

 This is useful when you are trying to retrieve information about a MIME
 type that doesn't exist in the datastore, but you have a file extension
 for that type, and nsIHandlerService::getTypeFromExtension returns another
 MIME type that does exist in the datastore and can handle that extension.

 For example, the user clicks on a link, and the content has a MIME type
 that isn't in the datastore, but the link has a file extension, and that
 extension is associated with another MIME type in the datastore (perhaps
 an unofficial MIME type preceded an official one, like with image/x-png
 and image/png).

 In that situation, you can call this method to fill in the handler info
 object with information about that other type by passing the other type
 as the aOverrideType parameter.

 @param aHandlerInfo   the handler info object
 @param aOverrideType  a type to use instead of aHandlerInfo::type

 Note: if there is no information in the datastore about this type,
 this method throws NS_ERROR_NOT_AVAILABLE. Callers are encouraged to
 check exists() before calling fillHandlerInfo(), to prevent spamming the
 console with XPCOM exception errors.

 Get the MIME type mapped to the given file extension in the datastore.

 XXX If we ever support extension -> protocol scheme mappings, then this
 method should work for those as well.

 Note: in general, you should use nsIMIMEService::getTypeFromExtension
 to get a MIME type from a file extension, as that method checks a variety
 of other sources besides just the datastore.  Use this only when you want
 to specifically get only the mapping available in the datastore.

 @param aFileExtension  the file extension

 @returns the MIME type, if any; otherwise returns an empty string ("").

 Remove the given handler info object from the datastore.  Deletes all
 records associated with the object, including the preferred handler, info,
 and type records plus the entry in the list of types, if they exist.
 Otherwise, it does nothing and does not return an error.

 @param aHandlerInfo  the handler info object

 Save the preferred action, preferred handler, possible handlers, and
 always ask properties of the given handler info object to the datastore.
 Updates an existing record or creates a new one if necessary.

 Note: if preferred action is undefined or invalid, then we assume
 the default value nsIHandlerInfo::useHelperApp.

 @param aHandlerInfo  the handler info object