Part of the space between the firmament and Earth is the area in contact with the Earth that we call the atmosphere, or sky. This is where the birds fly. It's referred to as the open firmament.
The First Heaven
The Three Heavens
Given that the apostle Paul specifically refers to something called the third heaven it's not a stretch to conclude that there must also be a first and a second heaven. We need to know what they are.
Firmament: רָקִ֖יעַ – raqia
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 3
According to the context of our verse the open firmament is where birds fly, so it's the atmosphere or sky. We can consider it to be the first of three heavens.
December 1st
We refer to the life after this one as "heaven" because we haven't paid attention to what the Bible says. We'll eventually live in a new Jerusalem at the the location presently occupied by our Earth.