Synonyms for Firmness (same or very similar meaning)
WordNet sense 1 (the muscle tone of healthy tissue):
soundnessWordNet sense 2 (the trait of being resolute):
resoluteness, resolution, resolve, resolve, firmness of purposeWordNet sense 3 (the quality of being steady or securely and immovably fixed in place):
steadinessFrom the ODE community, based on WordNetadd/edit