Firmament, having the word “firm” in it, sounds to me like “land”. So In Genesis 1:6, when it says:
6 Then God said, “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” 7 Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so.
I thought that made perfect sense, divide the firmament from the waters; it must be land being separated from the waters (because remember the earth was covered with water).
8 And God called the firmament Heaven. So the evening and the morning were the second day.
So in the beginning, after the flood that takes place between Genesis verse one and Genesis verse 2 (when Satan fell from heaven, etc) the earth was formless and empty. It was so chaotic that when the third of the angels fell with Satan to the earth that it even mingled the atmosphere of heaven with earth, and water with land, and light with dark.
Dividing and separating the land from the water did not happen until the third day.
It’s amazing to me that no matter how many times I read the book of Genesis, God shows me new things!