There are two ways to physically confirm the existence of the firmament:

1. Fly Straight Up:

You could fly a drone or small aircraft straight up until it reaches the firmament. If you get high enough, you could reach out and physically touch it — assuming you’re allowed to go that far.

2. Travel to the Edge:

Take a boat toward the outer edge of our world. Look at picture #3 and #4 — the closest countries to the edge are Australia and Argentina. From there, you’d need to sail toward the white area, which is ice and snow. Once you reach the ice wall, you can travel along it toward the outer edge until you reach the firmament. The distance from the ice wall to the firmament is estimated to be about 10 km (7 miles).

It’s extremely cold, and there are no roads or gas stations out there, so you’ll need a vehicle with a large fuel tank — or a private plane, but most can only fly about 9,000 miles. Either way, it’s expensive and you’ll need permission, which most people never get.

People have tried. Some sent drones with cameras, but that’s considered illegal and they’re often intercepted. Authorities don’t want people getting too close.

Also, I think you're misunderstanding what I meant by "flat." When I say the Earth is flat, I mean it rests on a flat surface — not that the terrain itself is flat. There are still mountains, valleys, and all kinds of landscapes inside the dome. The earlier pictures only showed the layout of the continents, not the terrain.

Here’s a better example showing the actual terrain: