Look up and target the interactive ledge that you will e able to make it onto, using the hook. Walk through the ventilation shaft again and reach the exit from the sewers to return to the Park Row district. Go towards the Gotham Pioneers Bridge, and listen to the radio communications along your way. After you get there, the game will play a cutscene , during which Batman manages to divert the Firefly's attention.
Note - you do not need to go, straight away, to the point where you fight Firefly. What is even more, Batman may in the meantime go to the batcave (e.g. in order to talk to Alfred, or complete new challenges), because the option to travel there has not been blocked.