Could be an entry from the Password Manager, or the autofill option.

Click in the field, enter a character or two and STOP ! Using the arrow keys, highlight an unwanted entry. Then press Shift > Delete Repeat as needed.

Suggestions that come from the Password Manager always have a key symbol in front, so this is likely saved form data.

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history": "Remember search and form history"

See also:

• https://support.mozilla.org/en-US/kb/automatically-fill-your-address-web-forms
• https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms