ok, here's the explanation:
Gravity causes denser (heavier) air to sink below lighter (less dense) air.
Heat is expansionary and will tend to flow from high to low density areas.
Since on earth, under gravity, the low density areas are higher than the high density areas, fire tends to go upwards.
But in zero gravity, the air's equally dense everywhere. Fire doesn't have anywhere to flow to, because there are no areas of lower density, so it stays in its natural state, which is a sphere. If you installed a vaccum generator above the fire (say, a ducted fan that sucks air out of the room the fire's in) (after all, all a fan does is create an area of low pressure into which the air around it flows, which creates the breeze) then the fire will head toward the low pressure area generated by the fan, and will look pretty much like fire on earth. If you stuck the fan below the fire, you'd get upside down fire
