Hi tetras_rock! Mollies are actually brackish water fish (not saltwater, not freshwater, something in between), and you are right, they do like some salt in their water. I have used salt at the rate of about a teaspoon per gallon for them, you may want to work up to that. Keep an eye on your other fish when you are doing this, as some are not as tolerant of the salt as others, corys especially. I actually at one time acclimated some large sailfin mollies to a full-blown saltwater tank to see if they would help out with an algae problem I was having.
Also keep in mind that mollies need a good portion of vegetable matter in their diet. You can get this many ways,Spirulina tablets or algae wafers from your lfs, a good growth of algae in the tank, supplementing their regular food with grocery store veggies such as cucumbers, romaine lettuce, canned or frozen peas (squished). There are lots of folks on this forum with ideas for feeding fish, check out some of the posts in the archives.
Hope this helps!