From the Vulgate firmamentum, which is used as the translation of the Hebrew raki’a. This word means simply “expansion.” It denotes the space or expanse like an arch appearing immediately above us. They who rendered raki’a by firmamentum regarded it as a solid body. The language of Scripture is not scientific but popular, and hence we read of the sun rising and setting, and also here the use of this particular word. It is plain that it was used to denote solidity as well as expansion. It formed a division between the waters above and the waters below (Gen. 1:7). The raki’a supported the upper reservoir (Ps. 148:4). It was the support also of the heavenly bodies (Gen. 1:14), and is spoken of as having “windows” and “doors” (Gen. 7:11; Isa. 24:18; Mal. 3:10) through which the rain and snow might descend.
Firmament
On the second day of creation God said, “Let there be a firmament in the midst of the waters,” and called it Heaven. The word pictures the great vault of sky above the earth, where on the fourth day God set the sun, the moon and the stars as lights “for signs, and for seasons.” Psalm 19 turns that scene into praise: “the firmament sheweth his handywork.” Daniel looks ahead to the wise, who “shall shine as the brightness of the firmament.” Begin with Genesis 1:6-8, then read Psalm 19.
10 verses
Genesis
And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
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.
And God called the firmament Heaven. And the evening and the morning were the second day.
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:
And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.
And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also.
And God set them in the firmament of the heaven to give light upon the earth,
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.
Psalms
To the chief Musician, A Psalm of David. The heavens declare the glory of God; and the firmament sheweth his handywork.
Daniel
And they that be wise shall shine as the brightness of the firmament; and they that turn many to righteousness as the stars for ever and ever.
From Easton's Bible Dictionary
Matthew George Easton, 1897