Delete it from the Password Manager and save it correctly.

Or edit it using the Saved Password Editor extension. https://addons.mozilla.org/en-US/firefox/addon/saved-password-editor/

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 the mouse or 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