Firefox can remember form entries in general. I don't recall whether that saves usernames. Could you check the following dialog to see whether you have that feature turned on?

orange Firefox button or classic Tools menu > Options > Privacy

In order to view or change this option, change to "Use custom settings for history"

Do you have "Remember search and form history" enabled? If not, you could try it out and see whether it is more helpful than annoying.

Thank you very much... It worked.

Note that saved form data is not connected to a specific website in contrary to the Password Manager, but merely to the name (id) of the input form, so entries may be suggested in other forms with the same name on other web pages.