Firestarter is a three day job, so prepare for a long endeavor. The whole thing involves a feud with the Mendoza family of the Cartel, and it starts with their weapons shipment.
Your first objective will be to locate the weapons. You'll want to stay undetected as long as possible, so be very cautious as you approach the hangar on the other side of the map. Once you reach it, you'll have several weapon sets to steal or destroy. Some are on the tables, some are inside the trucks.
If you intend to steal the weapons, you'll have to hoist them all back to the van. Daisy chain as best you can without alerting the guards around - the back alleys with walkways work best. Be careful and quick, and you can be in and out with little trouble. This approach will also net you more money.
If you're going to destroy the weapons, be prepared for danger. To destroy the weapons, you must use cans of fuel lying around the hangar. You can use this to burn the weapons, but be aware that this will net you less reward money. It will also attract far more attention and resistance.
This job gives you plenty of spots for holding out against the cops: in the hangar, behind the hangar, and in the back alleys. If you need to stand your ground, throw down a Doctor Bag and an Ammo Bag in one of these spots and dig in.