I've decided both the standard Kent Hovind/Creationist Canopy theory and Rob Skiba's Dome theory are wrong.
Even if you can convince me of a Flat Earth cosmology for other reasons I still won't think Firmament refers to a Dome. Even if you can convince me there is a Dome for other reasons, I still won't believe the word Firmament refers to that Dome.
I don't care how many Hebrew lexicons and concordances say it's something solid. Your standard Hebrew experts today don't think Almah means Virgin.
The word is rarely used, The Torah uses it only in Genesis 1, two Psalms use it, Ezekiel uses it and Daniel 12:3. The rarer a word is the more dependent we are on using Scripture itself to help define it. So yes it's presumed to derive from words for beaten metals, but it's about equally similar to the Hebrew word for perfume.
Fortunately God foresaw what this word means being an issue and told us. Genesis 8:1 "And God called the firmament the Heavens", the very origin of the word "Heavens" was to be a synonym for the Firmament. I know Kent Hovind doesn't like this fact, but Heavens is plural every-time it occurs here. We know there are at least three Heavens because Paul describes a Man (common theory is it's himself) being taken to the third heaven in 2 Corinthians 12:2.
Rob Skiba makes a good point that when Beth is used as a prefix it means "in". But he doesn't think the sun and moon (maybe he does the stars) or the birds are "in" the firmament, he thinks they're under it.
The birds fly in the first heaven, the atmosphere, the sun, moon and stars are in the second heaven, space.
The Firmament is also defined as what separates the waters above from the waters below.
When Ezekiel uses the word in his vision of God's Throne, it's hard for me to make sense of it with that being the same Firmament whatever your cosmology is. He has the Cherubim under it but God's throne above it.
Update January 2016: Rob Skiba just did a video obsessing over how much Water is in the Atmosphere, to serve his Flat Earth agenda. Never occurred to him to consider that this is the Water above of Genesis 1.
Judges 5:4 shows that the Biblical Authors knew that Rain came from The Clouds. Likewise Psalm 77:17.