I am surprised that this option doesn't already exist. Every time I log into my local library's website, Firefox asks if I want it to save my payment info. Problem is this isn't a payment card, it's just my numeric library card number. The only options are 'not now' and under a dropdown one can find a 'never remember cards' option. This is a little extreme - I don't want to never be asked about genuine credit cards again, but I am sick of being asked if I want to save my library card as a credit card.