Question: Does Genesis 1:6 really say that there is a "firmament" or solid structure, like a dome over the earth?
Response: No. In fact, the text is very clear that it isn't referring to a solid object.
The word that is sometimes translated into English as "firmament" is the Hebrew word "raqiya." The word "raqiya" appears nine times in the first chapter of the Bible's book of Genesis. In each of these cases, the "raqiya" is described as something that separates one thing from another, such as the waters below from the clouds above, or the ground below and the stars above.
People sometimes jump to the conclusion that this thing, or layer, that separates one thing from another must be a solid object. And that is faulty reasoning. The fact is, air is what separates the ground beneath our feet and the clouds above our heads. And the fact is, air is not a solid object.
Genesis 1:6 refers to the air that is between the ground and the sky. This is reinforced in Genesis 1:20, when the Bible speaks of birds flying in the "raqiya."