Why does FB reject the 6 digit code this app gives me?

[DaLeSy]

I'm trying to use this for Facebook and FB gives me a QR code. I click the google authenticator plugin icon at the top of firefox and it lets me copy the QR code. It takes it and labels it Facebook. Then gives me a 6 digit code to give back to FB. But when I copy and past it into FB, FB rejects it. And then I try again and the dang six digit number has changed. And keeps changing ever so many seconds or something. But still FB doesn't accept them, why?