1. The projectile is the flame rtardant.
2. The recoil is the extinguisher being pushed back.
3. The force on the flame rtardant is equal to the force on the extinguisher. Newton's Third Law says that the action must have an equal and opposite reaction.
4. The mass of the flame rtardant is lesser than the mass of the extinguisher. The flame rtardant is foam, while the extinguisher is solid metal.
5. The acceleration of the flame rtardant is greater than the acceleration of the extinguisher. Since F = ma, and the forces are equal but the mass of the flame rtardant is lower, the acceleration of the flame rtardant must be higher to match the same force.