What exercises will firm and tone your breasts?

Chest exercises and over the head with your arms type of exercises may help some, but nothing short of surgery will make them stand up like they did when you were younger/before you had kids. Which ever the reason is for you.

Here are more opinions and answers from other FAQ Farmers:
I'm pretty sure any pectoral muscle exercises gives them more definition, but they essentially are fatty tissue.
Try taking 10lb weights in each hand. (10 might be a bit much to start with. whatever seems comfortably difficult). Spread your arms and bend them so your forearms are perpendicular to the ground and your upper arms are parallel with it. Bring your hands together using your whole arm. Then you move your arms together so your elbows touch. Do lots of reps until it's a little firmer in the area above and beside your breasts then you can move up in weight. When I say a lot of reps, I mean a lot. You're gonna be real sore the day after you do it, so don't be alarmed.
Your breasts themselves have no muscle tissue so you cannot firm them up but if you do exercises to firm up the supporting pectoral muscles you can give them support from behind.
Push-ups are a main exercises in my routine. The tend to keep my breasts perky. So yeah, push-ups.