It's not flat in the physical way you think. Think of the earth as a physical dimension in which time is linear and matter is in a physical form. Then think of the firmament as the barrier or window to a dimension where it's the opposite and time is 3 dimensional and matter is on dimensional. You need to travel the speed of light to enter that dimension. It's why we can't leave earth in the traditional way you think. It's also why the stars ever seem to move and how the big bang happened instantly.