firmness vs possession

firmness and possession both are nouns.

Word	Noun	Adjective	Verb	Adverb
firmness	Yes	No	No	No
possession	Yes	No	No	No

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

firmness (noun)	possession (noun)
the quality of being steady or securely and immovably fixed in place	anything owned or possessed
the trait of being resolute	the act of having and controlling property
the property of being unyielding to the touch	(sport) the act of controlling the ball (or puck)
the muscle tone of healthy tissue	the trait of resolutely controlling your own behavior
	a territory that is controlled by a ruling state
	a mania restricted to one thing or idea
	being controlled by passion or the supernatural