Many Tupperware products are safe to use in the microwave, even some of those that predate the common use of the a microwave. However, it is best to use products specifically designed for use in a microwave.
Besides the potential destruction of the container due to melting, there is the concern about food contamination from plastics migrating to food heated in the microwave. The Food and Drug Administration tests different kinds of containers and food wraps to determine the amount of plastics that transfer into microwaved food. It is best to verify product use of any container before using it in the microwave.