We were there in January and had to wait 50 minutes after our ADR time to get a table (this was an hour before fireworks) and people were standing along the walls waiting for a table; the lounge was SRO. Even in January, this is a popular place, especially close to fireworks time. It's much easier to get into the lounge as a walkup when the place opens.
If it's really important for you to be up there for fireworks, I would make an ADR for an hour before the fireworks and then when you get upstairs, request a table in the lounge if possible. Otherwise, there is a chance they will be at room occupancy and will not allow you up there.