How does one stop Firefox asking to update password on secondary signin pages?

Question

Many websites use both a user name/password and a 'secret question' for sign in. The second part usually involves entering the answer to a randomised question, like 'mother's maiden name'; 'place of birth' etc.
Whenever this happens, Firefox asks if I want to update the password it stores. Obviously I don't because it will be changing my actual password, and the answer will be different the next time anyway. This happens on every sign in and there's no way I can see to tell Firefox not to; the only options are to update or 'not now'
I still want Firefox to remember the main user name and actual password, so how do I prevent it from asking about these pseudo passwords without disabling its recording of the main login details?

Answer 1

Thank you, philipp, that did the trick. I'm not sure that should be the preferred way of handling the problem though! Surely Firefox is clever enough to make some distinctions.....
Ah well. Had better spend a little time blocking all my affected sites.
Thanks again.