Definition of firmaments:
(n) :
(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.
(n) :
(countable) The field or sphere of an activity or interest.
(n) :
(uncountable, astronomy, historical) In the geocentric Ptolemaic system, the eighth celestial sphere which carried the fixed stars; (countable, by extension) any celestial sphere.
(n) :
(uncountable, obsolete except biblical) The abode of God and the angels; heaven.
(n) :
(countable, obsolete, also figuratively) A basis or foundation; a support.
(n) :
(countable, obsolete) A piece of jewellery worn in a headdress with numerous gems resembling stars in the sky.