Instructions
Head to the crate marked on the mini-map after you receive your orders from Captain Espinoza.
Walk next to the crate to add fire bottles to your inventory.
Press and hold the "L2" button to bring up the weapon wheel.
Select the fire bottle icon in the top-left section of the weapon wheel using the "Left Analog Stick."
Release the "L2" button. Marston will put away his currently held weapon and bring out a fire bottle.