I’m not sure about that interpretation, @Reggie_O_Donoghue
Genesis 1:8 says God called the Firmament Heaven… but then, in rapid succession, we get four verses in the same chapter that uses the following language:
Gen 1:14
And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:
Gen 1:15
And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.
Gen 1:17
And God set them in the firmament of the heaven to give light upon the earth,
Gen 1:20
And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.
So I’m not quite sure what you are trying to say:
If the term Firmament was perfectly equivalent to Heaven, the scribe would not have to say:
“in the open Sky of the Sky”.
The “Firmament” OF the Sky suggests that Firmament and Sky share the equal trrait of being “lofty” or “high” …
but the Firmament is still different.