That type of data is stored in form history. You can either clear individual entries or the entire set of form history. https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms#w_deleting-individual-form-entries and https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms#w_clearing-form-history

Hi, if the details are being stored in Firefox, you can remove them by deleting all references to the website in question, as shown under the heading 'How do I remove a single website from my history?' in this article - Delete browsing, search and download history on Firefox, then try the site again.

If they are still there, it's likely that they are stored on the website you are using, so you would need to see if there is a delete option in the settings, or contact the site owner for advice.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

Use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list with the mouse or cursor Down key
   do not click the mouse or press the Enter key
3. press the Delete key (on Mac: Shift+Delete) to delete the highlighted entry