The word is mentioned in the King James Bible, in the course of the creation story of (Genesis 1:6–8):

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.

The Jewish Encyclopaedia describes the Firmament as follows:

"The Hebrews regarded the earth as a plain or a hill figured like a hemisphere, swimming on water. Over this is arched the solid vault of heaven. To this vault are fastened the lights, the stars. So slight is this elevation that birds may rise to it and fly along its expanse."2

The book of Genesis goes on to mention lights being placed in the firmament (Genesis 1:14-17):

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: the stars also.

The Sun and Moon were thought to move in and out of the Firmament dome through a series of openings (reflecting the apparent movement of their rising and setting points throughout the year). This is explained in considerable detail in the Book of Enoch (the following is an excerpt):

This is the first commandment of the luminaries: The sun is a luminary whose egress is an opening of heaven, which is (located) in the direction of the east, and whose ingress is (another) opening of heaven, (located) in the west. I saw six openings through which the sun rises and six openings through which it sets. The moon also rises and sets through the same openings, and they are guided by the stars; together with those whom they lead, they are six in the east and six in the west heaven. All of them (are arranged) one after another in a constant order. There are many windows (both) to the right and the left of these openings. First there goes out the great light whose name is the sun; its roundness is like the roundness of the sky; and it is totally filled with light and heat. The chariot in which it ascends is (driven by) the blowing wind. The sun sets in the sky (in the west) and returns by the northeast in order to go to the east; it is guided so that it shall reach the eastern gate and shine in the face of the sky (1 Enoch 72:2-5).

Biblical references to this cosmology (specifically, the notion of a solid Firmament with Heaven above it) include the creation of the Firmament in Genesis 1:6; God opening windows in the Firmament in Genesis 7:11 to let water rain down, and closing them again in Genesis 8:2; the construction of a tall tower to reach Heaven in Genesis 11:4; celestial warehouses for snow and hail in Job 38:22, the sky as a strong crystalline material in Job 37:18 and Ezekiel 1:22; the sky as a tent in Isaiah 40:22; stars as small objects attached to the Firmament (which can fall off) in Daniel 8:10, Matthew 24:29, Mark 13:25, Revelation 6:13, Revelation 8:10, Revelation 9:1 and Revelation 12:4 (it is sometimes claimed that these "falling stars" are meteors, but the swipe of a dragon's tail dislodges "one-third of all the stars in the sky" in Revelation 12:4).