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Here is as good a meaning as you will get.

The firmament is the curve of the sky, especially if you imagine it as a solid surface. You can describe the sky at night as a firmament shining with stars.

The word firmament comes from the Latin firmus, or "firm," and this description of the sky as something solid reflects ancient ideas of the way the universe was constructed. The first stargazers imagined the sky as a sphere, 

My understanding of the firmament is the space created as the universe. Both FEs and GEs know that outside the Earth is that space. As for the waters well the clouds for one are waters and they are separated from the part of the firmament between the Earth and the clouds. There is no big mystery to this as some make out. These same waters would be used by God to flood the whole Earth.

So, let us see this in the light of the private revelations to Sister Mary of Jesus, known as Mary of Agreda (1602-1665), a nun known to have bilocated over 500 times to America without leaving her convent in Spain. The following are insights, dictated to her, she said, by the Virgin Mary herself in 1637, a mere four years after Galileo’s trial. Her three-volume work was entitled; ‘The Mystical City of God’ also known as ‘The Divine History and Life of the Virgin Mother of God.’ These revelations to Venerable Maria, whose body now lies miraculously incorrupt in a Franciscan Monastery in Spain, have withstood many years of investigation and even misinterpretation that caused them to be placed on the Index in 1681 but lifted after 3 months of inspection before receiving approbations from popes as a way to gain a deeper understanding of the Catholic faith in line with traditional Church teaching.

Mary of Agreda wrote: 'In the same instant, and as it were in the third and last place, God determined to create a locality and an abode, where the incarnate Word and his Mother should converse and dwell. For them primarily He created the heaven [the firmament] and Earth with its stars and elements and all that is contained in them . Of the first day Moses says that “In the beginning God created heaven and Earth.” And before creating intellectual and rational creatures, desiring also the order of executing these works to be most perfect, He created heaven for angels and men, and the Earth as a place of pilgrimage for mortals. These places are so adapted to their end and so perfect that as [King] David says of them, the heavens publish the glory of the Lord, the firmament and the Earth announce the glory of the work of his hands (Ps.18:2)… Of the Earth Moses says that it was void, which he does not say of the heavens, for God had created the angels at the instant indicated by the word of Moses: “God said: Let there be light, and light was made.” He speaks here not only of material light, but also of the intellectual or angelic lights… God created the Earth co-jointly with the heavens in order to call into existence hell in its centre; for, at the instant of its creation, there were left in the interior of that globe, spacious and wide cavities, suitable for hell, purgatory and limbo. And in hell was created at the same time material fire and other requisites, which now serve for the punishment of the damned.'