firmament vs heavens

firmament and heavens both are nouns.

Word	Noun	Adjective	Verb	Adverb
firmament	Yes	No	No	No
heavens	Yes	No	No	No

As nouns, firmament and heavens are synonyms defined as:

firmament and heavens: the apparent surface of the imaginary sphere on which celestial bodies appear to be projected

Other synonyms of firmament include celestial sphere, empyrean, sphere, vault of heaven, welkin.

firmament (noun)	heavens (noun)
the apparent surface of the imaginary sphere on which celestial bodies appear to be projected	the apparent surface of the imaginary sphere on which celestial bodies appear to be projected