noun

• (usually uncountable, literary, poetic, also figuratively) The vault of the heavens, where the clouds, sun, moon, and stars can be seen; the heavens, the sky.
  "And thei that be wiſe, ſhal ſhine, as the brightnes of the firmament: & they that turne to righteouſnes, ſhal ſhine as the ſtarres, for euer and euer."
• (countable) The field or sphere of an activity or interest.
  "the international fashion firmament"
• (uncountable, astronomy, historical) In the geocentric Ptolemaic system, the eighth celestial sphere which carried the fixed stars; (countable, by extension) any celestial sphere.
  "[B]etweene the ſphere of Saturne and the Firmament, there is ſuch an incredible and vaſt ſpace or diſtance (7000000. ſemidiameters of the earth, as Tycho [Brahe] calculates) void of ſtarres: [...]"
• (uncountable, obsolete except biblical) The abode of God and the angels; heaven.
  "Praiſe ye the Lord. Praiſe God in his Sanctuarie: Praiſe him in the firmament of his power."