Let’s look closer at what the Bible says about the firmament.
Gen 1:6-8 (KJV)
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.
The Hebrew word for firmament is raqia.
Outline of Biblical Usage defines it:
A. expanse (flat as base, support)
B. firmament (of vault of heaven supporting waters above)
In the Septuagint, the word is stereoma.
Outline of Biblical Usage:
1. that which has been made firm
A. the firmament, the arch of the sky, which in early times was thought to be solid
B. that which furnishes a foundation
C. firmness, steadfastness
In the Latin Vulgate, the word is firmamentum.
mid-13c., from Old French firmament or directly from Latin firmamentum “firmament,” literally “a support, a strengthening,” from firmus “strong, steadfast, enduring” (from suffixed form of PIE root *dher- “to hold firmly, support” ).Used in Late Latin in the Vulgate to translate Greek stereoma “firm or solid structure,” which translated Hebrew raqia, a word used of both the vault of the sky and the floor of the earth in the Old Testament, probably literally “expanse,” from raqa “to spread out,” but in Syriac meaning “to make firm or solid,” hence the erroneous translation.
https://www.etymonline.com/word/firmament
The Hebrew word raqia has one definition that is lost in the Greek and Latin translations, the concept of expanse.
Many modern translations understand this and use the word expanse instead of firmament.
CSB
Then God said, “Let there be an expanse between the waters, separating water from water.”
HCSB
Then God said, “Let there be an expanse between the waters, separating water from water.”
NASB
Then God said, “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.”
NET
God said, “Let there be an expanse in the midst of the waters and let it separate water from water.”
NIV78
And God said, “Let there be an expanse between the waters to separate water from water.”
WEB
God said, “Let there be an expanse in the middle of the waters, and let it divide the waters from the waters.”
If we look at other passages using raqia, the connotation is an expanse, rather than any firm structure.
(Gen 1:20) And 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.
(Ezek 1:22) And the likeness of the firmament upon the heads of the living creature [was] as the color of the terrible crystal, stretched forth over their heads above.
(Ezek 10:1) Then I looked, and, behold, in the firmament that was above the head of the cherubims there appeared over them as it were a sapphire stone, as the appearance of the likeness of a throne.
As I mentioned before, we know the Greeks believed in solid spheres to explains the stars, planets, and sun. And more than likely, it was Greek cosmology that influenced the rendering of raqia in the Septuagint translation into stereoma.
Around the 4th to 3rd centuries BCE the Greeks, under the influence of Aristotle who argued that the heavens must be perfect and that a sphere was the perfect geometrical figure, exchanged this for a spherical Earth surrounded by solid spheres. This became the dominant model in the Classical and Medieval world-view, and even when Copernicus placed the Sun at the centre of the system he included an outer sphere that held the stars (and by having the earth rotate daily on its axis it allowed the firmament to be completely stationary).
https://en.wikipedia.org/wiki/Firmament
When the Latin Vulgate came out, the Ptolemaic view of the universe was fully embedded into thinking and led to using the word firmamentum.
Ptolemy goes beyond the mathematical models of the Almagest to present a physical realization of the universe as a set of nested spheres
https://en.wikipedia.org/wiki/Ptolemy
And with the early English translations, almost all of them translated it as firmament, due to the influence of the Septuagint and Latin Vulgate.
Bishops
And God said: let there be a firmament betwene the waters, and let it make a diuision betwene waters and waters.
Coverdale
And God sayde: let there be a firmament betwene the waters, and let it deuyde ye waters a sunder.
Geneva
Againe God saide, Let there be a firmament in the middes of the waters: and let it separate the waters from the waters.
KJV1611
And God said, Let there be a firmament in the midst of the waters: and let it diuide the waters from the waters.
Tyndale
And God sayd: let there be a fyrmament betwene the waters ad let it devyde the waters a sonder.
Wycliffe
And God seide, The firmament be maad in the myddis of watris, and departe watris fro watris.
So, what I argue is the idea of a “firmament” was not introduced by the original Hebrew authors, but by the influence of the Greek cosmology during the translation of the Septuagint and later with the Latin Vulgate.
https://debatingchristianity.com/forum/viewtopic.php?p=1084793#p1084793