Try clearing history and ticking "Form & search history".

Thanks, zeroknight. Your response did cure my problem, but when I restored "Form & search history" setting but when I restored my History, but bad entry came back to life when I tried to sign-in. Thanks, John.

You can use these steps to remove saved items from an autocomplete drop-down list.

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

Note that hovering with the mouse doesn't work.

• https://support.mozilla.org/en-US/kb/remove-websites-address-bar-suggestions
• https://support.mozilla.org/en-US/kb/firefox-suggest

Disable form autofill by changing History to "Use custom settings for history" and untick "Remember search and form history".