The English word "firmament" in the Bible is a translation of the Hebrew, raqia, meaning "expanse." Its meaning is not "firm boundary" as Biblical critics have alleged, but might be better paraphrased as "stretched-out thinness" or simply "space."
Its first occurrence in the Bible relates it to heaven: "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 called the firmament Heaven" (Genesis 1:6,8). This firmament obviously could not be a solid boundary above the sky, but is essentially the atmosphere, the "first heaven," the "space" where the birds were to "fly above the earth in the open firmament of heaven" (Genesis 1:20).
There is also a second firmament, or second heaven, where God placed the sun, moon, and stars, stretching out into the infinite reaches of space. "And God set them in the firmament of the heaven to give light upon the earth" (Genesis 1:17).
The firmament in our text, however, is beneath the very throne of God, and above the mighty cherubim (Ezekiel 1:23) who seem always in Scripture to indicate the near presence of God. This glorious firmament, brilliantly crystalline in appearance, must be "the third heaven" to which the apostle Paul was once "caught up" in a special manifestation of God's presence and power, to hear "unspeakable words" from God in "paradise" (II Corinthians 12:2-4).
All three heavens "declare the glory of God" and all three firmaments "sheweth His handiwork" (Psalm 19:1). Therefore, we should "Praise God in His sanctuary" and also "praise Him in the firmament of His power" (Psalm 150:1). HMM