The firing sequence of internal combustion engines is the sequence of ignition for the cylinders in spark ignition, eg. For gasoline/petrol engines, the firing order corresponds to the order in which the spark plugs operate.
In diesel engines, the firing order corresponds to the order in which fuels are injected into each cylinder. Four-stroke engines must also time the valve openings relatives to the firing order, as the valves do not open & close on every stroke.
Firing order is the order of power distribution of each cylinder in a multi-cylinder reciprocating engine. This is achieved by placing the spark plugs in the correct sequence in a gasoline engine or by the sequence of fuels injection in types of diesel engines.