stiffness vs firmness

stiffness and firmness both are nouns.

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

As nouns, firmness is a hypernym of stiffness; that is, firmness is a word with a broader meaning than stiffness:

Other hypernyms of stiffness include firmness of purpose, resoluteness, resolution, resolve.

stiffness (noun)	firmness (noun)
the physical property of being inflexible and hard to bend	the quality of being steady or securely and immovably fixed in place
the property of moving with pain or difficulty	the trait of being resolute
the inelegance of someone stiff and unrelaxed (as by embarrassment)	the property of being unyielding to the touch
excessive sternness	the muscle tone of healthy tissue
firm resoluteness in purpose or opinion or action