The firmness lasts what, all of 30 seconds?....
You body is just reacting to the cold, trying to keep itself warm and the effects won't last long at all.
The only way to tone is to work the muscles.
You can work your pectoral muscles which are under the muscle for a slight 'lift' effect but since breasts are all fat you can't exercise them.

Like stopped for a few years and then it picked up again?

I'm 23 and happily an A cup!

....but it's fun to have bigger breasts for a short time