I have discovered that I can get a gasifier burn with a horizontal firebox if there is a baffle or part of a baffle in the stove to make the smoke go to the front of the stove to get out.
New wood or wet wood is put to the back of the stove, the hot coals and burning wood is raked to the front next to the front draft.

I prefer a vertical stove because.
A horizontal firebox delivers most radiant heat more up and down.
A vertical firebox delivers more radiant heat sideways.

A horizontal stove laying down in the corner of the tent does not deliver as much radiant heat to the bodies in the tent as a vertical stove standing in the middle of the tent.