First, i'd like to say congratulations on your weight loss. Keep at it!
To actually firm / fill out your breasts there is not much you can do other then prohormones which will fill them out. More specifically, estrogen and progesterone which is normally secreted by the ovaries and the placenta will increase the development of your milk-secreting glands (which are in your breast tissue). In effect, it's the same thing which happens when you get pregnant. The developed glands will fill out your breasts. This is why birth control pills can increase breast size and firmness---you are tricking your body into believing it is pregnant for a short period of time, which triggers the queue to begin developing more estrogen and progesterone. Prohormones will be a constant development and will help much more than birth control pills would.
Your other option is to remove the fatty tissue behind your breasts. Cardiovascular exercise is one way of doing this. Also body cream with aminophylline has been been proven to reduce the fatty tissue in your breasts as well. An inexpensive alternative.. It will help, but it won't turn you into pamela anderson. Exercise with weights will help a bit, but again...Not drastically.
Your final ditch effort is to increase your protein intake. Protein plays a leading role in the creation of callogen.
Remember to cocoa butter and vitamin e your breasts daily to avoid stretch marks while you're losing weight.
Hope this helps,
justin