Actually aquarium salt can still benefit mollies eventhough they've been raised in freashwater.
Mollies in their natural habitat can be found in brackish water OR (more commonly) in fresh water streams.
The reason aquarium salt benefits these fish is not because of the salinity itself but rather the hard minerals it provides them and also their water with (to work as a buffer)
ideal pH for mollies is 7.5 - 8 so slighty hard and alkaline.
also a huge benifit is that it's an effective anti-parasite and antifungal medication when used regularly in the tank (with water changes and what not)
mollies can be prone to lots of nasty diseases. so my suggestion is to add a tablespoon of a aquarium salt for every 5 Gal. (which is certainly not too much) along with every water change. Your mollies will be happier, healthier, and be around for you longer than they would without it.
hope this helps