The Hebrew word translated as "firmament" is raqia. A simple definition is "expanse" so most translations render the meaning as "the expanse of the sky."
Accepting the firmament as a sphere of rigid crystal on the edge of space solves many scriptural and physical problems. The King James Version is the only Bible from which an accurate cosmological model may be deduced.
Then I looked, and, behold, in the firmament that was above the head of the cherubims there appeared over them as it were a sapphire stone, as the appearance of the likeness of a throne.
One of our favorite ways to study the Bible is to follow a doctrine through from the first time it's mentioned to the last. This is a great way to develop understanding of the doctrine in the broad narrative of scripture.