I think this is the first time these have helped me.
I first heard the word firmament in a Rammstein song and when I translated it it was the same in English so I looked it up the only definition I could find was just "the sky".
It’s, yes. Back when they thought the sky had to be solid because something was holding it up.
Early Christian’s, for example believed that the water for Noah’s flood came from the firmament melting (since that bit in genesis describing the sky as “the waters” etc.)(it was written by Bronze Age peeps. Not exactly at the height off science.)
Yeah in the context of the song it more so refers to the sky above the horizon (the part which seemingly curves around the earth). Thinking about that I guess that is also what the picture is talking about