firing vs onset

firing and onset both are nouns.

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

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

Other hypernyms of firing include attack, onrush, onslaught.

firing (noun)	onset (noun)
the act of setting something on fire	(military) an offensive against an enemy (using weapons)
the termination of someone's employment (leaving them free to depart)	the beginning or early stages
the act of discharging a gun
the act of firing weapons or artillery at an enemy