https://support.mozilla.org/en-US/kb/automatically-fill-address-and-credit-cards

https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms

That could be the form filler built into Firefox. 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.

You could also try out Private Browsing - Use Firefox without saving history

Firefox Private Browsing is great for viewing websites without saving things like cookies, temp files, and a history of the pages you visit.

https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing

Thank you for your answer, Fred.

Your suggestion with Shift + DEL indeed deletes them from the list, but is there a way to stop Firefox saving any data at all, so I don't need to keep deleting same stuff all over again? I use forms like that on my day job, so Incognito mode isn't really an answer for me. Names and numbers stack up very quickly, so I find Firefox suggesting any inputs at all only very annoying.

Type about:preferences#privacy<enter> in the address box. Look for Forms and Autofill

Or you can try out Private Browsing posted above.

If you disable the login manager then Firefox can use use saved form data to store the username.

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