To shoot 4th July fireworks in New York City there are manly 3 solution, shoot from Brooklyn or east side to have the Manhattan skyline as foreground and fireworks in background, shoot from the Hudson on the west side or shoot from New Jersey as to have the Manhattan Skyline in the background.
Weehawken is a perfect spot to have the skyline in the background. There are many place from where to shot but from Hamilton Avenue the view is perfect, the Empire State Building is just in front and on the right side there is the Freedom Tower.
Go to Weehawken from Manhattan is very simple and cheap. There are shuttle bus to and from 8th Av, between 40th and 41st street (just in front of New York Times Building) or on the 42nd street on the side of Port Authority Bus Terminal (PABT), the cost is 3$ and the service operate 24hours every day, there also scheduled bus from and to the PABT.
It's important to get there very early, 4/4:30 pm, carry something to drink and eat during the waiting hours but don't worry there are public toilets.
I suggest to use a wide angle (around 24mm) for a panoramic view as to capture fireworks from the 4 barges. But I also suggest to use a multifunction lens like a 24/200 or 28/300 to zoom on fireworks over landmarks like the Empire State Building and the Freedom Tower. If you have a wide angle and a zoom like me, need to switch lens in the middle of the show (you have time, it's a 25 minutes show), so be ready to do this.