Of the 17 times the word firmament is used in the Bible, over half (or 9) of the times are found in Genesis One. Firmament means an expanse or expansion of space and it refers to the expansion of space between the earth and God's heavenly abode. It is bound by the waters above and the waters below (Genesis 1:6-8 [6] And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. [7] And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. [8] And God called the firmament Heaven. And the evening and the morning were the second day.
See All...). This explains why there are "waters that be above the heavens" (Psalm 148:4Praise him, ye heavens of heavens, and ye waters that be above the heavens.
See All...). Of the three heavens indicated in 2Corinthians 12:2I knew a man in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth;) such an one caught up to the third heaven.
See All..., the firmament includes the first two heavens while the third heaven refers to God's dwelling place. The first heaven is the "open firmament of heaven" where the birds fly (Genesis 1:20And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.
See All...). The second heaven is "firmament of the heaven" where the planets and stars reside (Genesis 1:14-15 [14] And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years: [15] And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.
See All...). As known to anyone who has considered the heavens as the work of God's finger (Psalm 8:3When I consider thy heavens, the work of thy fingers, the moon and the stars, which thou hast ordained;
See All...), the firmament exalts God. As the psalmist proclaims in Psalm 19:1The heavens declare the glory of God; and the firmament sheweth his handywork.
See All..., "The heavens declare the glory of God; and the firmament sheweth his handywork."