hey, and welcome to 'ology!
adding muscle will make you more compact and dense, which will firm you up. it will also make you leaner, since muscle cells take up less space than fat cells. just think: would you rather be flabby or tight? contrary to popular belief, it isn't that easy for women to get huge and bulky (thus the use of Anabolic Androgenic Steroids (AAS) in some female bb'ers). hope that helped! try lifting for a month or two; you'll be amazed at how much better your body will look!
~MINX