Scripture indicates that it didn’t rain until the time of Noah’s flood. Before then, plants received moisture from a kind of mist (see Gen. 2:5 and 6). Why was the rainbow such a big deal to Noah? Because one had never appeared before, because it had never rained before.
A reasonable theory about this “firmament” is that it’s exactly what Scripture says — a layer of water at the edge of the atmosphere. This water layer (or vapor canopy) collapsed after “the fountains of the great deep burst forth” (Gen. 7:11). (Regarding “fountains of the deep,” consider that there’s more water beneath the mantle of the earth than in all the oceans of the world.)
Interestingly, during the time this vapor canopy enveloped the earth, people lived hundreds of years. After that point, they came to live fewer than 100 years.
All this to say that the biblical concept of “firmament” is just what it says, and has fascinating repercussions in regards to lifespan, the Great Flood, and so on.
I am desperately trying to remember the name of a book I read many years ago that had exactly the same thesis you are describing.
I know I read it, I recall many of the details, but the title and author are gone from my memory.
Another interesting item, I remember learning, is that before the flood people were vegetarians. It is only AFTER the flood, that God commanded Noah and his family to begin eating meat. That would correspond to the need for complete proteins that exposure to the unfiltured-by-a-canopy-of-water sun depleted in the human body and which also shortened their lifespans.