According to Isaac Newton's first law, if we throw anything upwards, it falls down due to gravitational force. But smoke doesn't fall down because it is a colloid - a substance that is microscopically distributed throughout another substance. It is a combination of airborne liquid, solid and gas. When smoke is emitted, it blends with the air in the atmosphere and hence becomes part of it.
Further, since the density of smoke is less than that of the surrounding air, it rises. It is the same with a balloon, which also rises as the air trapped in it has less density than that of the surrounding air.