Why don't creationists teach a literal firmament anymore?

Many (perhaps most or all) creationists don't believe in a physical barrier or firmament because they don't believe the texts of the Bible were actually trying to communicate that. They believe that although those texts were originally meant to be understood as describing reality and as accurate records of history, they used figurative language in those descriptions. They believe that when the cosmology of Genesis is described as having a 'firmament' it is being used figuratively to describe how God structured the universe, rather than indicating that there is a real physical barrier.

In general, the genre of a text and the presence or absence of figurative language, neither requires nor prohibits the text from describing historical events. Consider Psalm 106 which tells the history of the Israelite nation through song, and the parables which though told like any other story, are clearly not intended to be understood as history. This article from Creation.com describes the general principles creationists use when interpreting Genesis:

• Should Genesis be taken literally?

Additionally, this article looks at the Hebrew work translated 'firmament', and suggests that it was never even meant to be thought of as solid, and that that idea was tacked on later.

• Is the raqiya‘ (‘firmament’) a solid dome?

In answer to your question let's take a look at what the Bible tells us as compared to we do know from known true Science.

From the Bible:

Gen 1:1 and 2 KJV

In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.

From Science (Hubble observations at http://hubblesite.org/newscenter/archive/releases/2013/20/full/)

A visible-light image, taken with Hubble's Space Telescope Imaging Spectrograph, reveals clumps of material in a circumstellar disk of gas and dust swirling around a 2- to 4-million-year-old star called AB Aurigae. These clumps, also composed of dust and gas, may represent the seeds of planet formation. Studying developing stars in the 1- to 10-million-year-old age range, such as AB Aurigae, could provide an evolutionary missing link in the planet formation process. "These observations suggest that the transition from gas and dust disks to debris disks with planets occurs around stars that are 1 to 10 million years old," explains Carol Grady of the National Optical Astronomy Observatories, working at the NASA Goddard Space Flight Center in Greenbelt, Md. "We have seen the beginning and the end of the planet formation process. Recent Hubble telescope observations have shown protoplanetary disks encircling young stars up to 1 million years old and planetary disks surrounding more mature stars around 8 to 20 million years old. Now we want to see what is happening in the middle."

So far what the Bible says and what Science has learned through interstellar observation coincide exactly

From the Bible:

Gen 1:3 through 5 KJV

And God said, Let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness. And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.

From Science:

The astronomers deduced the dust size from the way the disks allow light to pass through them. The fine dust normally seen in space scatters blue light but allows red light to pass through. The Sun appears red at sunset because atmospheric dust influences light in the same way as space dust. The dust disks in Orion, however, appear gray because they allow all colors of light to pass through. This is unusual in space and can only be explained if the dust is much larger than interstellar dust. Radio observations also provide tantalizing hints that much of the material in the disk may range in size from snowflakes to gravel.

So light is non existent in massive dense interstellar dust clouds but can pass through as these particles condense into clumps, and when the clumps get large enough all light is allowed to pass through.

Sounds like what the Bible said to me.

But let's jump ahead to the water part, which is what you asked about in the first place.

From the Bible:

Gen 1:6 and 7 KJV

And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.

From Science:

SEPTEMBER 7, 2005: Observations of Ceres, the largest known asteroid, have revealed that the object may be a "mini planet," sharing many characteristics of the rocky, terrestrial planets like Earth. Ceres' mantle, which wraps around the asteroid's core, may even be composed of water ice. The observations by NASA's Hubble Space Telescope also show that the asteroid has a nearly round shape like Earth's and may have a rocky inner core and a thin, dusty outer crust. Astronomers enhanced the sharpness in these images to bring out features on Ceres' surface, including brighter amd darker regions that could be asteroid impact features. The observations were made in visible and ultraviolet light between December 2003 and January 2004.

So massive amounts of underground water seem to be fairly normal, and now to do we need a glut of water in the Sky somewhere in order to have 40 days of Rain?

From the Bible:

Gen 6:17 KJV And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die.

Gen 7:4 KJV For yet seven days, and I will cause it to rain upon the earth forty days and forty nights; and every living substance that I have made will I destroy from off the face of the earth.

Gen 7:11 KJV In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.

Gen 7:12 And the rain was upon the earth forty days and forty nights.

From science: (http://www.fastcompany.com/1769468/scientists-discover-oldest-largest-body-water-existence-space)

Scientists have found the biggest and oldest reservoir of water ever--so large and so old, it’s almost impossible to describe. The water is out in space, a place we used to think of as desolate and desert dry, but it's turning out to be pretty lush. Researchers found a lake of water so large that it could provide each person on Earth an entire planet’s worth of water--20,000 times over. Yes, so much water out there in space that it could supply each one of us all the water on Earth--Niagara Falls, the Pacific Ocean, the polar ice caps, the puddle in the bottom of the canoe you forgot to flip over--20,000 times over. The water is in a cloud around a huge black hole that is in the process of sucking in matter and spraying out energy (such an active black hole is called a quasar), and the waves of energy the black hole releases make water by literally knocking hydrogen and oxygen atoms together. The official NASA news release describes the amount of water as “140 trillion times all the water in the world’s oceans," which isn’t particularly helpful, except if you think about it like this. That one cloud of newly discovered space water vapor could supply 140 trillion planets that are just as wet as Earth is. Mind you, our own galaxy, the Milky Way, has about 400 billion stars, so if every one of those stars has 10 planets, each as wet as Earth, that’s only 4 trillion planets worth of water. The new cloud of water is enough to supply 28 galaxies with water. Truly, that is one swampy patch of intergalactic space.

So if there is a lake of water that large, could it just be possible that a much smaller lake might have crossed the Earths orbit at the same time as the Earth was at that periphery at the same time, could it also be possible that at the same time the Earths surface opened up and water gushed to the surface?

Might just be possible, but it seems far more likely that some greater power caused them all to happen simultaneously, that is just supposing that it happened that way.

But the most stupefying question to me is How did Moses know without all of these modern Sciences imagine such an unheard of event?

People don't seem to believe in the firmament as a literal barrier of water up in the sky that collapsed at Noah's flood.

The answer to this is amazingly simple.

Many Young-Earth Creationists believe that there was a layer of water of some form (ice, water, other possibilities) in the upper atmosphere, or in space, surrounding the earth AND Young-Earth Creationists believe that there is a Firmament.

However, almost none believe that the Firmament is a solid layer of water because YEC-ers have a hyper-literal understanding of the Genesis account. And...

It's impossible to get the idea that the firmament is a solid layer of water from the Genesis account.

That's the answer. Here's the explanation:

From Genesis 1 (KJV)

6 And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.

7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.

8 And God called the firmament Heaven. And the evening and the morning were the second day.

Jumping down to verse 20:

20 And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.

The Firmament is the place between the waters - it divides the waters above from below. It's not a solid layer of water, it's the space between the waters. It's where the birds fly. It'd be a bit hard for birds to fly in a solid layer.

So, again, the reason Young Earth Creationists don't teach that the firmament is a solid layer of water is that YEC-ers are very literal in the interpretation of Genesis (by definition) and there's absolutely no way to get the idea that the firmament is a solid layer of water from the text.

The idea, however, that the layer of water (in whatever form it took) was part of the source of water for the flood (along with the fountains of the deep) is pretty common among Young-Earth creationists, however, and is taught widely.

Details on the nature of the waters vary. Some say clouds or water vapors. Others think a layer of ice in the upper atmosphere or in space. There's not much agreement on what form it took, but it is widely believed to have existed in some form.