A fire can be compared to a substance that is being vigorously oxidised. The fire wouldn't ascend up as long as the material didn't.

The flame and the air are both being pulled downward by gravity. The cooler, denser air exerts pressure on the lighter flame when it is moved because of its density. The flame is forced upward by the pressure, which is greater at the bottom than at the top. That is buoyancy; it is also what causes boats to float.

The cooler, denser air does not move downward when gravity is eliminated. It keeps applying pressure on the flame from all sides, causing it to emerge as a sphere rather than moving up.

What are some of your favorite science phenomenon?