Hi Sanguine,
Fire_outbound_plug method is used in window outbound plug and that to only when you want to navigate between two components which are added in a main component( in which a navigation link is defined for the two components added in the component usage ).
If at all there are two components and the second component is added to the first component component usage and defined a navigation link then in the window outbound plug we directly use view_manager->navigate( ) method.
Regards,
Krishna.