firing vs combustion

firing and combustion both are nouns.

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

As nouns, combustion is a hypernym of firing; that is, combustion is a word with a broader meaning than firing:

Other hypernyms of firing include burning.

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