hi, you can select the information in the dropdown-field with the arrow keys on the keyboard and then hit the del-key in order to delete that particular piece of information.

Okay that worked, thank you. Is there any way to know where that information was stored, in case there's other information I don't want saved (but won't really think about til I see it autofill)?

firefox will store this data in the formhistory.sqlite file within your profile folder. however that's in a database format, so it's not trivially easy to open it and display its contents.

it's possible to clear all existing autofill entries in one go though: Control whether Firefox automatically fills in forms