Tsk, tsk.
Remember your high school physics?
Air, in a sense, moves randomly, due to the fact that atoms are always moving. This principle is called diffusion. Since diffusion is present in most substances, this means that a continual supply of oxidizer will be present. Fires in zero-G burn longer, due to fewer oxygen needs, and diffusion keeps them supplied with oxygen.
For more information, I would suggest looking at the May '98 issue of Scientific American.