nsIContentHandler in Firefox 3.5

 Tells the content handler to take over handling the content. If this
 function succeeds, the URI Loader will leave this request alone, ignoring
 progress notifications. Failure of this method will cause the request to be
 cancelled, unless the error code is NS_ERROR_WONT_HANDLE_CONTENT (see
 below).

 @param aWindowContext
        Window context, used to get things like the current nsIDOMWindow
        for this request. May be null.
 @param aContentType
        The content type of aRequest
 @param aRequest
        A request whose content type is already known.

 @throw NS_ERROR_WONT_HANDLE_CONTENT Indicates that this handler does not
        want to handle this content. A different way for handling this
        content should be tried.