All righty, then. You say there's a firmament that holds the atmosphere to keep it from "escaping". On the other hand, atmospheric pressure is about 14.7 PSI at sea level. At the top of Mount Everest, it's about 4.9 PSI, because there's less air pressing down on it from above, firmament or no. Any 3 mile column of air has as much air as the remainder of the atmosphere above it. If the atmosphere were constant density throughout, it'd be only 5 miles thick, and obviously, airplanes fly higher than that. How high is this firmament exactly, and wouldn't the pressure be so low that it doesn't actually need to be "contained"? TL;DR The atmosphere isn't "next" to a vacuum at all.