Open a new file I used 300x100, press D to reset your colours.
Goto Filter > Render> Clouds, then Filter > Render > Difference clouds keep pressing Ctrl+F until you get something that's sort of like fire such as:
Then use the brush tool to go over parts at the top to make it look like the fire ends somewhere and doesn't just take up the whole image such as:
Then goto Filter > Liquify using a size 15 brush, with 50-60 density, 100 brush pressure. to sharpen the ends and curve them to a more fire like effect.
Then goto Image > Adjustments > Gradient Map
Change the colours to this:
and there you have it, a fire like effect, simply....its all down to the difference clouds really..
More Tutorials: 
