Firestarter has us trying to steal some weapons from the Mendozas gang, so cut the fence and make your way down to the hangers.
I'm not covering stealth for this day, so charge down guns a blazing. One of the four hangers will be open and filled with the weapon shipment we're looking for.
The weapons are held in these Black cases, so open the up and bag all the weapons. There will be quite a few cases scattered around in the hanger, so search thoroughly.
There are also some weapons hiding in the back of the trucks which need opening.
Now you need to decide what to do with the weapons. If you just want to get out of here fast, find this fuel tank.
Unleash bullet hell on the tank until it explodes open like a fiery stove. Then start throwing all the weapons into it so they explode just like the tank did. Once you've blown enough up, escape will become available.
If you're a money hungry criminal though, you can carry the weapons all the way up the hill to your van and throw them in to take them for yourself. Obviously this gets you a bigger payout, but getting them to the van isn't the easiest, the weapon bags are really heavy.
Either way, steal/destroy enough and the escape will open.