Placing Styrofoam in the microwave is usually okay as long as you keep an eye on the Styrofoam and don't leave the Styrofoam in the microwave longer than a couple minutes.
I've place Styrofoam in the microwave before to just quickly reheat the food or a cup of coffee and as long as I don't leave the Styrofoam cup or Styrofoam container in the microwave longer than 2 minutes it's never been a problem.