combustion vs firing

combustion and firing both are nouns.

Word	Noun	Adjective	Verb	Adverb
combustion	Yes	No	No	No
firing	Yes	No	No	No

As nouns, firing is a hyponym of combustion; that is, firing is a word with a more specific, narrower meaning than combustion:

Other hyponyms of combustion include arson, fire-raising, incendiarism, ignition, inflammation, kindling, lighting, incineration.

combustion (noun)	firing (noun)
the act of burning something	the act of setting something on fire
a process in which a substance reacts with oxygen to give heat and light	the termination of someone's employment (leaving them free to depart)
a state of violent disturbance and excitement	the act of discharging a gun
	the act of firing weapons or artillery at an enemy