Can you remove the saved logins one by one, or is that disabled somehow?

You can stop Firefox from asking to save new logins on the Settings page. More info in this article: Password Manager - Remember, delete and edit logins and passwords in Firefox -- see the "Disable the Firefox password management feature" section.

Success?

But by the way, Firefox may still save usernames -- without the passwords -- in form history. This article has information on disabling form history: Control whether Firefox automatically fills in forms.

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