But if it is flat then when all the water spills off the top of the ice wall we'll all get pulled off the edges by the water flow.

Or, I dunno, does the firmament stop water? It has to, right? Otherwise all the air would also escape. In which case the water will just build up and up until the entire snow globe is filled to the top!

Then we end up in a Truman Show scenario where we can just literally sail to the edge of the Earth but not fall off because we hit the sky box I guess?