This add-on is very simple and has no configuration. If you enable the add-on, whenever you open a reply window and start composing a reply to only the sender of an email, it checks to see if there is a Reply-to listed in the header. If there is, it populates the "To" box with the address listed in Reply-to instead of the sender. That's the only thing this add-on does. That means if you respond to an email sent to a list, the default "Reply" will now automatically create an email to send to the whole list. If you only want to send to the original sender, you just need to change the "To" address.