A 'Fire tornado' or 'Firenado' or 'Fire whirl' are a fearful feature of forest fires world wide. Having seen one up close, I can attest that they are both awesome and utterly terrifying, but I am aware that these vortices are not strictly speaking, actual tornadoes.
An example, filmed in fires in 2012 around Alice Springs, Northern Territory, Australia, as in this YouTube Clip.
The Wikipedia page for this topic rather vaguely states
Often, fire whirls are created when a wildfire or firestorm creates its own wind, which can turn into a spinning vortex of flame.
But, the page does provide the following public domain image of a 'fire tornado':
How does the vortex of fire in a 'fire tornado' form?