Stop saving any form data or usernames

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.

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