Definitions from Merriam-Webster for
firmament:
a.) air
b.) basis
c.) the field or sphere of an interest or activity.
So were Gen 1:6-7 using this rather vague definition and then switched to primary definition [air] in verse 8 and thereafter?
Sounds like a fascinating question, but I’m not fully understanding it.
At times the word translated “firmament” in the KJV and other translations seems to be speaking of the expanse of the sky, the region under the “canopy” or “dome” at the edge of the atmosphere. At other times, it seems to be speaking merely of the vast region “up there” above us.
The ESV Study Bible explains:
“... it is difficult to find a single English word that accurately conveys the precise sense of the Hebrew term shamayim, “heaven/heavens.” In this context, it refers to what humans see above them, i.e., the regions that contains both celestial lights (vv. 14-17) and birds (v. 20).”
I did find the discussion of this concept at Wikipedia to be pretty fascinating: http://en.wikipedia.org/wiki/Firmament
Maybe in the end, we should just think of “firmament” broadly as “space,” distinct from “stuff.”