I think the hydroplate theory explains this water separation fairly well since firmament iirc could be defined as earth and/or air. see creationscience.com where the water separation in Gen 1:6-7 might imply subterranean water - water that later flooded the earth when the fountains of the great deep burst open [Gen 7:11] and then it rained [1st recorded rain in Genesis] for 40 days and nights.
Definitions from Merriam-Webster for
firmament:
a.) air
b.) basis
c.) the field or sphere of an interest or activity.
So were Gen 1:6-7 using this rather vague definition and then switched to primary definition [air] in verse 8 and thereafter?