The fire particles are a simple, yet important particle for ATF: Deep Six events. (rest in peace, pal.) They usually are in use if a tank or some form of artillery is firing or a building has been impacted by a bomb and is now burning out.
A list of the uses of them are:
Fort Shad - Artillery & blown up buildings.
July 21st Event - MEB Artillery and Soup Shark and post office.
Railtank - Railgun and "Main Defense Turret".
(Mid-air explosions not included).
USS Winfall - Hit turrets and firing turrets.
July 4th Event - JD (Justice Department) and Pharmacy being bombed by the B1.
MEB AA emplacement set up after the convoy which fires at the B1.
US Bomber - Wings and body (after it crashes).
Flamethrower - when it's fired, (though it uses a different particle).
Tank - Causes an explosion where the cannon is facing and a fire particle ensues for a set period of time.