Rakia, the word for firmament is literally the beating of metal into a thin foil. In the context of Genesis, which is the only place I can find the word used like this, I am convinced it is a description of the bounds of the universe.
It was translated firmament, because no one really knows what that is. Until recently, no one would dream to translate it the way I do.
The air seperated the firmament that was the seas from the firmament above. Pararphrasing, I'll dust the Bible off in a bit and read the passage. Firmament meant water. I have heard it said when the firmament above collasped, for lack of a btter word, the earth flooded.