The firmament, I believe, is the atmosphere.
The apostle Peter explains why the waters above the expanse are no longer there. Speaking of the time when God flooded the world, and correlating that with the time of Christ's return, he said...
2 Peter 3:3-7....
"First of all know this, that in the last days ridiculers will come with their ridicule, proceeding according to their own desires 4 and saying: “Where is this promised presence of his? Why, from the day our forefathers fell asleep in death, all things are continuing exactly as they were from creation’s beginning.”
5 For they deliberately ignore this fact, that long ago there were heavens and an earth standing firmly out of water and in the midst of water by the word of God; 6 and that by those means the world of that time suffered destruction when it was flooded with water. 7 But by the same word the heavens and the earth that now exist are reserved for fire and are being kept until the day of judgment and of destruction of the ungodly people."
It appears as if God used the water suspended above the atmosphere to flood the earth in Noah's day. It stood suspended there "by the word of God" so a scientific explanation would have no bearing on conjecture that it was impossible for it to be there. A water canopy surrounding the earth would have created "hot house" conditions on earth, evening out the climate and creating a temperate, humid environment conducive to lush growth. The fact that they have unearthed palm trees in Siberia lends credence to that situation IMO.
Jesus said his return would see world conditions similar to those times....violence and immorality being the norm....and here we are again.
And, according to Strongs Concordance, the word used for "circle" in Hebrew (chuwg) also means a sphere.