I would consider that 'firmament' may represent either the first heaven or the first heaven and second heaven combined as one 'expanse'.
You only say this because you have been taught "vastness-of-space" all your life - it is not a matter of [real] scientific proof.
It is not actually as obvious as you may think.
(BTW - Why are the sun, moon, and stars only literal in Genesis?)
How can you even know with absolute certainty just exactly how far the second heaven reaches?
Other than simply believing what you are told (acting on assumption and blind trust), you cannot know with absolute certainty - because you have not actually conducted your own scientific proof.
So then, it is really a matter of faith...
The text surrounding the sun, moon and stars make them literal. The heaven was literal, the earth was, the light was, everything was literal.
You said "How can you even know with absolute certainty just exactly how far the second heaven reaches?"
Gen 1:7 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.
Gen 1:7 says that there are waters below the firmament (seas) and waters ABOVE the firmament. So that firmament extends from the surface of the earth up to, but not into, the area where the clouds reside.
So what do we call the area ABOVE the firmament?
It's another firmament (expanse). This is the
firmament of heaven of which the lower parts of it contain water and expands outward for who knows how far. This, "the firmament of heaven," is where the sun, moon and stars were placed. Agree?