Actually, light, in the form of photons of ultra-high energy gamma and X-Rays formed very soon after the Big Bang, so the Bible (accidentally) sort of got this one right. Visible light did not appear until much later, but long before the formation of the first stars. So light, as an electromagnetic wave, is self-perpetuating and doesn't actually require a source.
Did the Genesis writers have some special insight that we have only now discovered? Not really. If you watch a sunrise or sunset, you will notice that the sky begins to light up long before the Sun becomes visible. Similarly, the sky will remain in twilight for some time after the Sun sets. So, the ancient writers of Genesis didn't really connect 'light' with the Sun. Since they weren't aware that the planet was spherical, they didn't realize that the Sun would light up the sky long before it appeared over the horizon.
Some of the prevailing theories of abiogenesis posit that the first life-forms derived their energy from hydrothermal vents, so it is possible that life (in the form of very simple algae) could have existed without sunlight. But, of course, it was the evolution of photosynthesis that really kicked evolution into high gear, since there was now orders of magnitude more energy available to drive evolution from simple algae, to the first cells and then plants and finally animals. So yeah - Genesis really dropped the ball on that one.
The scientific problems with Genesis go far deeper, though. For one, what exactly is this "firmament" thing? Creationists will tell us that is it simply our atmosphere. This becomes a little problematic in verse 16, though, where we are told that God placed the Sun and Moon in the firmament. Clearly, the Sun is not inside our atmosphere, and neither is the Moon. Aha! says the Creationist. The "firmament" must be a reference to the Solar System as whole. Well, in that case, what are the "waters above the firmament"? It's here that the Christian mental gymnastics become Olympic contenders. Perhaps the "waters above" are a reference to the Oort cloud, a belt of icy comets that surrounds the edge of the Solar System. But - water and ice are not the same thing. If the Bible had meant "rocky debris containing large amounts of water ice", then why didn't is just say so?
Other will tell us that the "waters above" referred to a thick vapor canopy that surrounded the earth and collapsed upon the planet during the Great Flood. A little bit of physics will soon dispel that notion. Genesis says that the water covered the highest mountains, and yet it only rained for 40 days. The amount of energy released by a deluge of that magnitude would have dwarfed even the 100,000 megatons released by the K-t event, and vaporized the earth.
So, what is the firmament? Quite simply, it's exactly what Genesis says it is. It is a solid dome that god constructed and placed into the infinite, eternal ocean (called tehom in Hebrew) in order to create a living space for humans. The "waters above the firmament: were exactly that - the ancient, primeval tehom that surrounded the earth, held back by the solid firmament. which also housed the Sun, Moon and stars.
The ancient Hebrew conception of the Cosmos was very similar to that of the Babylonian/Sumerian creation myths, which should not come as a surprise, since we now know that the Israelites and the Canaanites shared a common Semitic ancestor, and are closely related, genetically. The fact that they originally shared a common religion should also come as no surprise.