Jesus = Firmament. This is so because he is the only mediator between heaven and earth.
Earth (us) ———————- Firmament (Jesus-Mediator) ————————— (heaven) departed saints.
You can’t talk to dead saints because Jesus is in the way.
The firmament, and this needs some fleshing out from the Hebrew, is the boundary between heaven and earth (understanding, of course, that the Bible uses heaven in a multiplex sense). Jesus becomes the New Firmament. The Firmament in Genesis 1 is not simply the boundary between earth and sky, or earth and waters, but also the boundary between the waters above (which is the sea before God’s throne) and the waters below (which have yet to be gathered into seas). The Firmament in this sense is “above” us.
While speculative, this makes infinitely more sense of the hilasterion passages of the NT. Liberals and Eastern Orthodox hate the word propitiation because it suggests God gets mad. Conservatives are on stronger lexical ground. I think propitiation is a good translation of Romans 3:25. It becomes problematic, though, when we move to 1 John 2, where it says Jesus is the propitiation for the whole world. This comes very close to universal atonement.
Most good commentaries will say that hilasterion is (correctly) referring to the Mercy Seat above the ark. If so, then Jesus is the new mercy seat who takes to himself God’s great fireball to destroy evil. He is the Protective Covering between heaven and earth. This way hilasterion means nothing about universal atonement.
John Barach has an interesting take on this.
(2) Let there be a firmament in the midst of the waters (Gen. 1:6) & You shall not make for yourself a carved image … you shall not bow down to them nor serve them (Ex. 20:4-6).
The firmament is between the waters above and the waters below. The firmament includes everything we call “outer space,” since the sun, moon, and stars will be placed in the firmament on Day 4. The waters above the firmament reappear later in Scripture as the sea below God’s throne (e.g., Rev. 4:6). Thus the firmament is the barrier and the mediator between heaven and earth. It’s a veil, corresponding to the veil in the tabernacle. The tearing of that veil represents the rending of the mediator (Heb. 10:20).
The second commandment has to do with bowing to images in order to worship Yahweh through them. The images are false mediators. So both the second word in Genesis 1 and the second word in Exodus 20 have to do with mediation.