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."
Firmament: רָקִ֖יעַ – raqia
A Sea of Glass
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.