I believe it is within, or just outside of the firmament because of the shape of the light that is cast over the surface of our known plane-t. Turn a big glass bowl upside down on a table, turn out the lights, put a flashlight just outside the bowl and circle that flashlight into the bowl just outside of the bowl and watch the shape of the light that is cast into the area under the bowl. That shape seems to match the light shapes that we can see. Sometimes the light is circular, sometimes it becomes oblong. Crrow777 has some awesome demonstrations of this which also make a lot of sense.