firmness vs strength

firmness and strength both are nouns.

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

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

firmness (noun)	strength (noun)
the quality of being steady or securely and immovably fixed in place	the property of being physically or mentally strong
the trait of being resolute	an asset of special worth or utility
the property of being unyielding to the touch	permanence by virtue of the power to resist stress or force
the muscle tone of healthy tissue	the amount of energy transmitted (as by acoustic or electromagnetic radiation)
	capability in terms of personnel and materiel that affect the capacity to fight a war
	physical energy or intensity
	capacity to produce strong physiological or chemical effects
	the power to induce the taking of a course of action or the embracing of a point of view by means of argument or entreaty
	the condition of financial success