Create a new file (600x500). Fill the layer with White BackGround.
Then, use Fiter->Render->Cloud.
Then,use Filter -> render-> different cloud.
Press Ctrl+F several times until It look as real Fire.
Use Brush Tool to change half above color to Black.
Use Filter -> Liquify to make the Fire more likely
Image -> adjustment ->Gradient map, set as follows:
You can select other Stop color to change color of fire.
Result:
