firmness vs stability

firmness and stability both are nouns.

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

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

firmness: the quality of being steady or securely and immovably fixed in place
stability: the quality or attribute of being firm and steadfast

Other hyponyms of firmness include granite, sureness, stableness.

firmness (noun)	stability (noun)
the quality of being steady or securely and immovably fixed in place	the quality or attribute of being firm and steadfast
the trait of being resolute	the quality of being enduring and free from change or variation
the property of being unyielding to the touch	a stable order (especially of society)
the muscle tone of healthy tissue