strength vs firmness

strength and firmness both are nouns.

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

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

strength (noun)	firmness (noun)
the property of being physically or mentally strong	the quality of being steady or securely and immovably fixed in place
an asset of special worth or utility	the trait of being resolute
permanence by virtue of the power to resist stress or force	the property of being unyielding to the touch
the amount of energy transmitted (as by acoustic or electromagnetic radiation)	the muscle tone of healthy tissue
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