As I do not practice in New Hampshire, I am not familiar with your local laws. However, generally when a parent signs away their parental rights to a child, they are no longer responsible for any support of that child. So in your case, if your child's mother wants her husband to adopt the child, you would no longer have to pay child support.
One issue that you may want to consult an attorney on is whether or not you would be able to see your daughter if you sign away your parental rights. Generally, signing away such rights includes the right to parenting time with the child. This is something that you will have to consider.