Firefox's Form History feature usually will display matches in a drop-down below a form field. However, if you have not visited the site before, then I suspect that will not work. You might need an add-on (an add-on you really trust!) to manage and fill card numbers and other form data.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a form filler.

HOWEVER, I would not advise this method.

It would be best to store such information in a text file and access it when needed.

I think that credit card numbers are among form data that is automatically excluded from being stored as save form data (Luhn algorithm).

• Bug 188285 - Form autocomplete should not store credit card numbers