See [https://support.mozilla.org/en-US/kb/control-firefox-automatically-fills-in-forms#w_prevent-firefox-from-storing-form-entries this section] in the support article > [[Control whether Firefox automatically fills in forms with your information]].

Use these steps to remove saved (form) data from a 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 #Press the Delete key (on Mac: Shift+Delete) to remove it. *http://kb.mozillazine.org/Deleting_autocomplete_entries ---- You can remove saved Password(s) here: * Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords" *http://kb.mozillazine.org/Password_Manager ---- Websites remembering you and automatically log you on is stored in a cookie.<br /> So you need to remove the cookies from that site to reset this choice. *Tools > Options > Privacy > Cookies: "Show Cookies"