Quote:

this could be taken to mean he opened the firmament


You cant 'open' the firmament, and you will notice that the Bible never states that the firmament was opened during the flood.

The firmament itself holds the clouds up because it is the air resistance which keeps the clouds in the sky, turbulence and air currents keep the clouds up though they can be filled literally with hundreds of tons of water.

The theory you learned was most likely the 'water vapor canopy theory' which has been a popular belief among creationists. I would agree with that theory to a point.