Biblical Use
The word is used in the Genesis creation narrative:
Then God said, “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” Thus 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. So the evening and the morning were the second day.
An extremely literalistic interpretation of the Bible and non-canonical related texts present a cosmology that is incompatible with modern scientific knowledge. The firmament was a great tent-like ceiling made of solid crystalline-like material, which, according to the pseudepedigraphic 2nd or 3rd century book of 3 Baruch, might be pierced by tower and gimlet. It had many windows, some of which opened and closed for the sun and moon to travel through or to let water, which was held above, fall through as rain. On top there were also warehouses of snow and hail. Stars were small objects that were attached tenuosly to its surface.
The Jewish Encyclopedia 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.
Read more about this topic: Firmament