As
@Heyo said, the 'firmament' is imagined as a hard dome above the (flat) earth, with the stars affixed to it in such a manner that if they come loose, they'll fall to earth.
The word is from Latin 'firmamentum' (from 'firmus', meaning 'firm'). It first appears in the Vulgate, translating the Septuagint's στερέωμα (from στερεόϛ, 'firm'). The Hebrew word being translated is
raqiya`, for which I find the following translation note:
1. extended surface (solid), expanse, firmament
─ expanse (flat as base, support)
─ firmament (of vault of heaven supporting waters above)
─ considered by Hebrews as solid and supporting 'waters' above
In other words, it means the sky, thought of as solid eg
Job 22:12 “Is not God high in the heavens? See the highest stars, how lofty they are! 13 Therefore you say, ‘What does God know? Can he judge through the deep darkness? 14 Thick clouds enwrap him, so that he does not see, and he walks on the vault of heaven.’
Job 37: 18 Can you, like him, spread out the skies, hard as a molten mirror?
Job 38: 7 when the morning stars sang together, and all the sons of God shouted for joy? [ie the stars are adjacent to each other on the fixed dome]
Isaiah 14:13 You said in your heart, ‘I will ascend to heaven; above the stars of God I will set my throne on high; I will sit on the mount of assembly in the far north; 14 I will ascend above the heights of the clouds, I will make myself like the Most High.’
Ezekiel 1:22 Over the heads of the living creatures there was the likeness of a firmament, shining like crystal, spread out above their heads. 23 And under the firmament their wings were stretched out straight, one toward another; and each creature had two wings covering its body. 24 And when they went, I heard the sound of their wings like the sound of many waters, like the thunder of the Almighty, a sound of tumult like the sound of a host; when they stood still, they let down their wings. 25 And there came a voice from above the firmament over their heads; when they stood still, they let down their wings. 26 And above the firmament over their heads there was the likeness of a throne, in appearance like sapphire; and seated above the likeness of a throne was a likeness as it were of a human form.
Ezekiel 10:1 Then I looked, and behold, on the firmament that was over the heads of the cherubim there appeared above them something like a sapphire, in form resembling a throne.
Amos 9:6 who builds his upper chambers in the heavens and founds his vault upon the earth,
Matthew 24:29 “Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken;
Revelation 6:13 and the stars of the sky fell to the earth as the fig tree sheds its winter fruit when shaken by a gale; 14 the sky vanished like a scroll that is rolled up, and every mountain and island was removed from its place. 15 Then the kings of the earth and the great men and the generals and the rich and the strong, and every one, slave and free, hid in the caves and among the rocks of the mountains, 16 calling to the mountains and rocks, “Fall on us and hide us from the face of him who is seated on the throne, and from the wrath of the Lamb;
For more on biblical cosmology, see
>here<.