Microwaving Styrofoam containers and cups is okay for as long as a couple minutes and if you microwave the Styrofoam longer than a couple minutes then it can start to melt.
Just keep an eye on the Styrofoam cup or container that you're microwaving so that you can remove it before it actually melts.
I've never had a problem microwaving any Styrofoam as long as I only microwaved it for 1 or 2 minutes and just kept an eye on it.
Any longer than a couple minutes then the Styrofoam container can start melting a hole through it which is not good for the food you're heating up.