You can use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page 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 the Delete key (on Mac: Shift+Delete) to delete the highlighted entry

Note that hovering with the mouse no longer works.

You can also check this in the Password Manager.

• Options/Preferences -> Privacy & Security: Logins: "Saved Logins" -> "Show Passwords"
• https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-passwords

Thank you, but I do not have a drop-down list for this user i.d. It is a governmental entity, and I have to enter a 13-digit number for my user i.d. Unfortunately, when I positioned the cursor at the end of the black dots, "Show" showed up. It showed me my password. When I cleared that and typed in the 13-digit number and exited the system and went back, the incorrect i.d. pre-populated the user i.d.

If editing a saved login doesn't work, completely delete it and then save it again after your next login.

I think that if you clear the entire item and then either double-click the line (maybe first set focus elsewhere) and then possibly type the first three numbers that Firefox will show the autocomplete drop down list with the suggestion. Then you should be able to remove this item via cursor down and the delete key. Editing an item that is prefilled is usually not a good idea anyway.