Tennessee Homestead, I wonder if you entered the number into a form field that for some reason Firefox did not recognise as "special", and so it ended up being saved as a standard autofill item?

You can delete all saved autofill form entries by using the "Clear Recent History" item from the top History menu, ensuring that the option "Saved form info" is checked and the time range is set to "Everything".

(But of course make sure that none of the other options such as browsing history or cookies etc. are checked, else they will be deleted also)

If you do not want to remove all form history, just the card number, then I personally do not know if that is possible? Perhaps you could try arrowing down to it in the dropdown list and pressing delete? Or right-clicking it and seeing what, if any, options come up? Or maybe there is a delete icon next to the item on the dropdown?

Sorry, I do not use form history and so have no way to test the deletion of single entries myself, should you not want to lose all form entries.

P.S. See this article for information on individual profile files and the data that they store: https://support.mozilla.org/en-US/kb/about-your-important-data-and-their-files