heavens vs firmament

heavens and firmament both are nouns.

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

As nouns, heavens and firmament are synonyms defined as:

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

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

heavens (noun)	firmament (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