The issue at hand is not the meaning of "the firmament," especially since there is no way of knowing the exact nature of the pre-Flood climate, and how the hydrologic cycle worked.  It's pointless to speculate on something about which we can't really draw any concrete conclusions.  The real issue is whether or not the days of creation were literal, 24 hour days. The context along with comparison with similar expressions in the rest of Scripture, are overwhelmingly in favor of a literal reading.  If God had meant us to understand long ages of time, He could easily have inspired Moses to describe it that way.  We wouldn't even be having this discussion if it weren't for the fact that too many Christians have put their faith in infidel scientists rather than the infallible Word of God.