The short version:

1. Go into the Material Room.
2. Select the material you want to make reflective.
3. Create a Reflect node (right-click, New Node --> Lighting --> Ray Trace --> Reflect)
4. Plug it into the material's Diffuse_Color, Alternate_Diffuse, or Reflection_Color channel (your choice, depending upon the effect you wish to achieve).
5. Enable raytracing in your Firefly render options.
6. Render.