I keep mollies and I love 'em
Like shawmutt said, mollies are brackish fish, so they need (or at least will be happier) with some aquarium salt added.
Mollies also do great with algea flakes/tabs or other vegetation added to their diet. (I feed mine tropical flakes daily and algea wafers a few times a week-daily when there are alot of babies in the tank because it is SO cute to see the ball of tails when all the babies eat it)
IME mollies will have babies every 3 or 4 months, so if you have females and males together, expect to have baby fish in the future, even if they don't have them at first or you don't have any males b/c the females can store up sperm for a while.
I keep mine without a heater, but to do that well you'd have to do without air-conditioning b/c it would make the room colder and therefore the tank, so if you have an
AC, I'd stick with a small heater.
I personally have never had a molly try to get out of my tank, though they do tend to hang out at the middle or top parts of the tank.