The Collaborative International Dictionary of English v.0.48:

Firmament \Fir"ma*ment\, n. [L. firmamentum, fr. firmare to make
   firm: cf. F. firmament. See Firm, v. & a.]
   1. Fixed foundation; established basis. [Obs.]
      [1913 Webster]

            Custom is the . . . firmament of the law. --Jer.
                                                  Taylor.
      [1913 Webster]

   2. The region of the air; the sky or heavens.
      [1913 Webster]

            And God said, Let there be a firmament in the midst
            of the waters, and let it divide the waters from the
            waters.                               --Gen. i. 6.
      [1913 Webster]

            And God said, Let there be lights in the firmament.
                                                  --Gen. i. 14.
      [1913 Webster]

   Note: In Scripture, the word denotes an expanse, a wide
         extent; the great arch or expanse over out heads, in
         which are placed the atmosphere and the clouds, and in
         which the stars appear to be placed, and are really
         seen.
         [1913 Webster]

   3. (Old Astron.) The orb of the fixed stars; the most remote
      of the celestial spheres.
      [1913 Webster]

WordNet (r) 3.0 (2006):

firmament
    n 1: the apparent surface of the imaginary sphere on which
         celestial bodies appear to be projected [syn: celestial
         sphere, sphere, empyrean, firmament, heavens,
         vault of heaven, welkin]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "firmament":
   Caelus, air, azure, blue sky, caelum, canopy, canopy of heaven,
   cerulean, cope, empyrean, ether, heaven, heavens, hyaline, lift,
   lifts, sky, starry heaven, the blue, the blue serene, vault,
   vault of heaven, welkin



Easton's 1897 Bible Dictionary:

Firmament
   from the Vulgate firmamentum, which is used as the translation
   of the Hebrew _raki'a_. This word means simply "expansion." It
   denotes the space or expanse like an arch appearing immediately
   above us. They who rendered _raki'a_ by firmamentum regarded it
   as a solid body. The language of Scripture is not scientific but
   popular, and hence we read of the sun rising and setting, and
   also here the use of this particular word. It is plain that it
   was used to denote solidity as well as expansion. It formed a
   division between the waters above and the waters below (Gen.
   1:7). The _raki'a_ supported the upper reservoir (Ps. 148:4). It
   was the support also of the heavenly bodies (Gen. 1:14), and is
   spoken of as having "windows" and "doors" (Gen. 7:11; Isa.
   24:18; Mal. 3:10) through which the rain and snow might descend.