What is the most basic Biblical understanding of the nature of the firmament? Going back to original Hebrew texts where possible, what is the most basic understanding of the firmament (the substance/realm/space that God supposedly stuck between heaven from earth)?  I've read a lot of literature theorizing about the antediluvian world, creationist texts and such, postulating (guessing about) different natures for the "firmament".  But looking only at the texts, what could be learned about the Biblical/ancient-Hebrew understanding of the stuff?
The word "firmament" denotes something firm and strong, referring to the "dome of the heavens" that was believed to make up the top of the sky in King James's day.  It's a rather poor translation of the Hebrew raqiya, meaning "expanse".

Raqiya is derived from the root raqa, which refers to stretching or spreading something out; the term was used in reference to the creation of gold leaf, for example.  The idea is that God put something thin and spread out (the atmosphere) over the surface of the earth.