Before I gave it much thought, and realized that the Earth is actually a flat plane, I used to twist this passage to mean a water vapor canopy which only existed before the Flood. 
But that was just me.

And when it talked about the windows of heaven being opened to flood the earth, I thought it was figurative, was a bit embarrassed by it, etc.

But now that I'm a dreaded "Flat Earther"  I have no such embarrassment. It all makes sense, and I have an easier time accepting everything in Holy Scripture, especially everything talking about the earth, its four corners, the ends of the earth, ascending into heaven, descending into hell, etc.

Please explain this from Holy Scripture.

And God made a firmament, and divided the waters that were under the firmament, from those that were above the firmament, and it was so.

 [Genesis 1:7]

The air contains moisture. You can see this as fog or clouds. Since the atmosphere is parallel with the curvature of the earth, it looks like a dome or firmament from any spot on the earth because you cannot see the other half of the sphere.

God created the atmosphere of AIR for creatures to breath between the waters on Earth and the waters in the sky , the clouds.

Very sad, cassini.  You are constantly promoting the notion that heliocentrism is heretical because it contradicts the meaning of Sacred Scripture as unanimously interpreted by the Church Fathers.  Well, the Church Fathers were unanimous that the firmament was a physical structure that kept physical waters from inundating the earth, and not a metaphor for the air and "clouds".

You have to at least try to come up with something along the lines of what Matthew held (which is what I held as well until I came to the realization that the earth is flat), a water canopy, in order to avoid heresy by your own criteria.

Sure, the air is now a FIRMamentum.  Church Fathers unanimously reject your theory.  At least Sungenis at one point tried to come up with his Planck fabric thing, although at another time he said it was "space".  So is it the air or space?  Sungenis came up with space, rather than your "air", because he realized that the Hebrews and Church Fathers understood the air, i.e. atmosphere, to be BELOW the firmament.  They had a notion of 3 Heavens, the first being the air above us, the sky, etc., the second being the firmament itself, and the third being the Heaven where God dwells that is above air, the firmament, and the waters above the firmament.

So both your "air" and "space" are epic fails and by the criteria of the Holy Office which condemned heliocentrism as heretical, your theory is heresy.

Please explain this from Holy Scripture.

And God made a firmament, and divided the waters that were under the firmament, from those that were above the firmament, and it was so.

 [Genesis 1:7]

Cera asked the above question. It has nothing to do with a flat Earth, it is a simple question.
We live on the Earth today. We can see the waters of the sea and the clouds.
Thus the question asked above can be answered by way of the senses.
It explains where the air that is necessary for life came from,
Is there any other Scriptural explanation of this creation?

As for a flat Earth or a global Earth, well the science of geodesy, measuring the shape of the Earth, has been practiced now for over 300 years.

Giovanni Domenico Cassini, dfefender of geocentrism, was one of the most skilled surveyors of his era, a man the popes in Rome had surveying rivers and lands to prevent flooding. So, as a true empiricist, he decided to measure the curve of the Earth as well as he could for himself in order to determine the true shape of the Earth in order to prove Newton's bulging spinning Earth was not bulge-shaped globe.

  

 the Academy.King Louis XIV of France approved Cassini’s last great expedition. With the aid of his son Jacques Cassini (Cassini II) and others, he measured the arc of meridian (see above) from Paris north to Dunkirk and south to the boundary of Spain, and, in addition, he conducted various associated geodesic and further south astronomical operations that were reported to the Academy. The Northern hemisphere was found to be curved like the top of an egg. 

In 1735, financed by King Louis XV this time, one group went to Peru under Pierre Bouguer and Charles Marie La Condamine and a year later another group went to Lapland under Maupertuis. Their findings were that the Earth is a globe. Ten years later, after measuring two baselines, one 12.2 and 10.3 kilometres (7.6 and 6.4 miles), La Condamine and his group emerged from the Amazon jungle with their mission accomplished, again confirming the Earth is a globe.

 COORDINATE SYSTEMS USED IN GEODESY
BASIC DEFINITIONS AND CONCEPTS
By Tomás Soler  and Larry D. Hothem,  Member, ASCE

‘INTRODUCTION: The principal problem of geodesy may be stated as follows (Hirvonen 1960): “Find the space coordinates at any point P at the physical surface S of the Earth when a sufficient number of geodetic operations have been carried out along S.” Therefore, in order to know the position P, the definition of an appropriate frame to which these spatial coordinates refer is of primary importance.  

For years now I have challenged flat earthers to show how this science can be illiminated as nonsense.

For years now I have challenged flat earthers to show how this science can be illiminated as nonsense.

There's nothing of substance here, only gratuitous assertion.  For years we have challenged Globe earthers to explain the experiments that conclusively indicate the contrary.