Mollies are brackish water fish usually, and are mostly found in the freshwater systems. If you drip acclimate them for about 4-6 hours slowly increasing the salinity of the water, their system adapts. Since they are not atrue reef fish though, they dont know the dangers of anemones, crabs, etc so in a reef system they disapprear pretty quick.
Many people keep them in their sump or fuge to breed and add food to the tank through the fry. They are a nice safe fish that doesnt hurt the system in anyway