firmness vs inflexibility

firmness

noun
  • physical durability; rigidness (of material) 

  • security; steadfastness; good grip 

  • strictness; mental strength 

inflexibility

noun
  • The quality or state of being inflexible, or not capable of being bent or changed; unyielding stiffness 

How often have the words firmness and inflexibility occurred in a corpus of books? (source: Google Ngram Viewer )