firmness vs stableness

firmness and stableness both are nouns.

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

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

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

Other hyponyms of firmness include granite, sureness, stability.

firmness (noun)	stableness (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 property of being unyielding to the touch
the muscle tone of healthy tissue