I agree with others, this seems like a flat-earther belief that has gotten mixed with Christianity’s creation story.

I’ve heard of it before while browsing the internet, but not from any Christians I personally know. All the Christians I interact with (myself included) believe in a literal interpretation of the creation but nobe of us believe in a physical firmament. I will note that a single man I know denies the moon landing (for reasons I haven’t bothered to ask) but even he still believes in a spherical earth and heliocentric orbit.

My understanding of Genesis 1:6-8 is that the firmament mentioned is the earth’s sky or atmosphere itself, and not a physical barrier at the edge of the atmosphere. The easiest way to show this would be Genesis 1:20 where birds are described as “flying in the firmament of heaven”. If the firmament was a solid object, birds could not fly in it.