Mollies can do well in fresh or brackish. In nature they can be found in fresh to the open sea. I've always had better luck with salt in the water but I'm told that because I have soft water and they prefer harder water.
There's some information on mollies under the Brackish FACs:
http://www.aquariacentral.com/faqs/brackish/FAQ3.shtml#3g
There's some information on mollies under the Brackish FACs:
http://www.aquariacentral.com/faqs/brackish/FAQ3.shtml#3g