Moon Fairy is right, you should rest you muscles a minimum of 48 hours between workouts, and the method she suggests is one way to do it. You can also do full body workouts with 2 days of rest in between.
However, I really wanted to say that if you want to make progress, you need to keep lifting heavier. Your body gets used to exercise at any level and stops making progress. The only way to get stonger/more sculpted, is to keep increasing your weights. In my opinion - and I'm sure that of many others here - is that you don't need to do the Firm workouts as well as weight training. There are lots of weight training routines out there. Many of us here are currently using the New Rules of Lifting for Women
which emphasizes workouts that are related to function, i.e. they use the movements of everyday life and make us stronger and more functional. But take a look at some of the stickied threads, and read through some of the other threads, and don't be afraid to ask questions. A fit body is a beautiful body!