How do you make a fluid emit smoke and fire?

Since a mesh can only have a single fluid sim applied to it, it is not possible to have both a fluid and a gas simulation at the same time. Instead, you need to add an additional mesh that can follow the fluid simulation to host the gas (smoke and fire) simulation.

Start by setting up and simulating your fluid. Once this is complete, create a new mesh (eg, a sub-divided plane) and use the Shrinkwrap modifier to map this over the fluid. Add the smoke and/or fire simulation to the 'shrunkwrapped' mesh in place of the fluid mesh.

This can produce results similar to the following :

For simplicity I only rendered smoke but the same principle applies to smoke and fire. Also, you could add a particle system to emit particles of smoke/fire from the surface or change the smoke emission with a texture for added variability.