This is probably gonna be a rambling mess, so let's just get the terms out of the way so you have something to take away even if the rest doesn't make sense.

6 Again God said, Let there be a firmament in the midst of the waters, and let it separate the waters from the waters. - Genesis 1:6

What's described in Genesis 1:6 is when God separated the "waters" of the World.

World does not always mean Earth, mind you.

World in the old texts is closer to what we ascribe to the "Universe", "uni" meaning one and "verse" meaning word.

Universe = One + Word

More particularly, the "World" is understood to concern the components of the Universe in which Man operates.

https://www.etymonline.com/search?q=world

So, Earth is the physical realm, the World entails the doings of Man, and the Universe is the housing of the "All", thereby receiving the honorary title of the "One Word." Which is the Will of God.

Okay, now that fluff is out of the way, what is the Firmament?

Well, we see a separation into threes here. Physical, Mindful, and Spiritual.

Exoteric = Physical, Esoteric = Mindful, and Spiritual is itself but also considered the Will and Guiding Hand of God. Spiritual is immutable, and Humans have no 'current' capacity to change the terms therein.

We can interact and alter the Physical World and the World of Man, but we do not fully have the authority to alter the Spiritual World. Doing so would be like necromancy (contacting the dead) or making deals with demons and angels. It's a bad idea, and we lost that Divine Authority when Adam was cast out of the Garden.

The Garden is the hub; apoint where the three (Physical, Mindful, and Spiritual) come to a crossroads. It is there you find passage to God's Throne. Because Adam was cast out, we must toil in the fields and feel the pangs of labor in order to come to know Him.

But the Garden itself is not the Firmament. Rather, the Firmament describes the underpinning of these three. One becomes three.

"As the number Three grows out of the One, likewise the Seven grows out of the Three" - https://www.sacred-texts.com/sro/wta/wta08.htm

But what is that "One?"

Well, that's where we get to modern terms...

The Matrix AKA the Simulation

Every simulation, world, fantasy, etc. all has rules which must be followed. In Dungeons & Dragons, it's the game grid, you stats, turn incentive, priority, what moves you can do, etc.

There is also a lore which is 'strictly' adhered to.

In terms of King David, he refers to this as the Law and the Lord which imposes said Law.

In the Matrix movies, you have rules which come up as well. Operating procedures and "glitches" which have greater significance. Things like deja-vu with the black cat are some such "glitches" which lend themselves to the protagonists recognizing the Matrix code was altered and that Agents were coming. It's like getting goosebumps when a demon is near. More importantly, though, each interaction the Matrix can be seen through a stream of some glyphic language moving up a computer screen. It's the run-time code, like an immeasurably complex string of 0's and 1's.

That Code is known as the Matrix Scroll -- all those green glyphic letters scrolling constantly. It's a language the crew of the Nebuchadnezzar can read but Neo on the onset is clueless about. In fact, the first indication of his capability to read the Scroll was when he resurrected and became The One. At that point he could also alter that Code.

You getting why I opened with all these terms and comparisons now?

The Firmament can be seen through the Matrix Scroll, like a Looking Glass. The Old Alchemists just didn't have any other words for it. They didn't have movie magic and technology to get the point across. The "As Above, So Below" phrase is describing the Run-Time programming code of the Universe going on at all times. If you're in the Matrix, you can't see the Scroll. If you're out of the Matrix, you can.

Only The One can see the Scroll DURING RUN TIME. And so, he can predict things because he can read ahead. Because he's also inside the Matrix, he can use that knowledge unlike the ship's crew just watching and incapable of acting. Furthermore, he doesn't have to experience things like others. Instead, he can just cut to the end of where the code loop leads and prepare or alter the code as he pleases.

Coming back the three, the Matrix Scroll represents the Mindful World. The Matrix itself represents the Physical World. The "Land of the Real" AKA the Machine World represents the Spiritual World, where Agents represent Demons and the Unplugged Humans represent Angels trying to dispel the illusion and free the People from the menace which would use the Humans' bodies and minds as components in their vast mechanical breadboard. (Originally, Morpheus was supposed to hold up a microchip processor, not a battery to explain what the machines used humans for.)

The Matrix is a Modern Alchemical Primer for the concept of the Firmament, as well as the Christ Figure's place in it.

On the evening of the second day of the week, God willed to divide the heavens from the earth, that there might be luminaries and stars beneath the heavens to give light to this world, and that the heavens might be a dwelling-place for the righteous and the angels after the resurrection. God said, 'Let there be a firmament which shall divide the waters from the waters'; and straightway the waters were divided into three parts. One part remained upon the earth for the use of men, cattle, winged fowl--the rivers and the seas; of another part God made the firmament; and the third part He took up above the firmament. But on the day of resurrection the waters will return to their former nature. - The Book of the Bee

This is the Firmament

The Firmament the ancient name for what we call today "The Matrix."

It's a way to peer into the realm of the Divine and see as Spirits do; to use the Symbols in nature to construct an alphabet with which we may discern what is True and what is False. What is good, and what is bad.

What will bring success, and what will bring ruin.

One may look to the stars to find discernment. Others may gaze at clouds to determine when it will next rain. Cataloguing flowers as well lends itself to this Universal knowledge.

The Predictive nature of the world and spotting patterns and trends IS what we call "reading the Matrix Scroll." Once you are able to see and read this "Word of God" you gain the ability to read and alter the code at run time -- thus becoming one of the "Enlightened" individuals like Neo. Enlightenment, however, is just the first step. The next step is the state of Christos, among whom One alone is King of Kings, Lord of Lords; the Alpha and the Omega -- First among Man, as to be the Son of Man; a Servant to All.

The Savior.

When we speak of Prophecy in the Bible, Prophets are merely those who have been graced with the ability to read the Matrix Scroll, the Firmament, and offer up 'predictions' which are nothing more than simply being the first to hear the train whistle and telling everyone to get off the tracks before they get smacked. It's like those who spot the black cats glitching out.

God's Will is like the train, and it will absolutely smack you if you try to stand against it. The Wise among us know not only when the train comes, but why we stay off the tracks when it does. Only those with Authority can punch the brakes.

What it all really comes down to, in that frame of mind, is Causality and the If-Then statement.

If you do X, then Y.

You see a pattern in nature, you write it down. Next time that pattern arises, people will see you like a magician and prophet for predicting the natural outcome. When you collect a number of these patterns, you bind them together and call it a Bible -- a code by which to live by. The patterns we call Parables, Analogies, Metaphors, Allegories, etc. are all these patterns in nature whose outcome holds constant given the same inputs.

Which in Truth is the Word of God. Spoken in Nature, in the form of patterns denoted by symbols, and transcribed by Man for other men to live honest and fulfilling lives. The Bible itself has an acquired divinity so long as the Natural World continues to act as it always has. In other words, the Bible is the User Manual and Textbook for the Coding Language (Firmament) by which the User operates in the Machine.

You can see the Firmament in all things.

In the night sky, in the clouds, interactions between animals, carpentry, and even by playing with a bit of string. The Dynamics and relationships between things are constants by which we can create programs which run with our individual wills, just as the Universe operates the Will of God.