To elaborate a bit one what Loma posted, the fire extinguishers are intended to put out a fire in the engine nacelle - external to the actual engine - and do not have any effect on the engine itself. Most aircraft use Halon gas as the agent (some new installations use a more environmentally friendly powder similar to baking soda). Firing the extinguisher doesn't damage anything (although the powder based systems require some cleanup afterward).
Pulling the fire handle isolates the engine - fuel, electrical, hydraulic, and pneumatic. If the fire handle is restored to the normal position, the engine can be re-started although there are no procedures to ever do so (and if there really was a fire, you'd have to be either really desperate or daft to re-introduce fuel to the scene of the fire).