Clearing "Form and search history" is about legacy form fill and doesn't include login data (username and password) and newer data like addresses and credit cards. Suggestions in a login drop-down menu that have a key icon in front are login suggestions that you need to remove in the Password Manager. *https://support.mozilla.org/en-US/kb/password-manager-remember-delete-edit-logins Suggestions in a drop-down menu without a key symbol are autocomplete saved form data. Suggestions without a key icon are part of autocomplete saved form data that you should be able to remove by selecting this item with the cursor Down key and pressing Delete (Mac: Shift+Delete). *https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms