How do I prevent computer from remembering my credit card number?

Hello Robin. Hopefully this support article is what you need: https://support.mozilla.com/en-US/kb/Form+autocomplete In any case, your form autocomplete information is safe and will not be transmitted to outside your computer. Now, inside your computer (when someone else uses your Firefox profile), it's not as secure, but it's still fairly secure.

You can look at Autofill Forms: https://addons.mozilla.org/firefox/addon/4775