The Collaborative International Dictionary of English v.0.48:

Firmness \Firm"ness\, n.
   The state or quality of being firm.

   Syn: Firmness, Constancy.

   Usage: Firmness belongs to the will, and constancy to the
          affections and principles; the former prevents us from
          yielding, and the latter from fluctuating. Without
          firmness a man has no character; "without constancy,"
          says Addison, "there is neither love, friendship, nor
          virtue in the world."
          [1913 Webster]

WordNet (r) 3.0 (2006):

firmness
    n 1: the muscle tone of healthy tissue; "his muscular firmness"
         [syn: firmness, soundness] [ant: unsoundness]
    2: the trait of being resolute; "his resoluteness carried him
       through the battle"; "it was his unshakeable resolution to
       finish the work" [syn: resoluteness, firmness, firmness
       of purpose, resolve, resolution] [ant: irresoluteness,
       irresolution]
    3: the property of being unyielding to the touch
    4: the quality of being steady or securely and immovably fixed
       in place [syn: steadiness, firmness] [ant: ricketiness,
       unsteadiness]