AoE or Singletarget modes have nothing to do with Rain of Fire.
Rain of Fire is activated by you either manually via leftshift mouseover or automated mouseover.
Say you want to keep singletarget on a boss and want to put Rain of Fire up on adds, just hover with the cursor over the adds and hold leftshift.
Say you gonna AoE a pack that lives long, you activate FnB rotation with the keybind and the automatic RoF keybind and mouseover hover the pack.