Smoke is difficult to see through. Smoke contains particle of unburnt carbon that make it look black. Soot particles make it hard to see through smoke. These products arise as a result of inefficient combustion. Smoke is hot.
Hot gases rise because they are lighter then air.
Because smoke is usually much hotter than the surrounding air it tends to rise. If it can move through open doors it will move to the top of the house, and gradually fill it As smoke rises away from a fire it tends to draw in cool air that mixes with the smoke. Eventually there is more air in the smoke than products of combustion. This makes the smoke less hot, but there is much more smoke to deal with. Smoke is dangerous.