In Windows my autofill will update asking for updating the new login when a new login is used. Did you not see this notification to update your autofill login?

Suggestions in a login drop-down list 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 list 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