https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms

You can use these steps to remove saved items from an autocomplete drop-down list. #click the (empty) input field on the web page to open the drop-down list #highlight an entry in the drop-down list with cursor Down key<br>do not click the mouse or press the Enter key #press Shift+Delete (68+) or Delete to delete the highlighted entry Note that hovering with the mouse doesn't work. *https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms If this is about the other auto-fill feature then check Options/Preferences. *https://support.mozilla.org/en-US/kb/automatically-fill-your-address-web-forms