I can think of three possible sources for the old user name showing:

(1) A saved login in Firefox (2) Form history in Firefox (3) A cookie that Google set for the site long ago

For #1, you can delete the old saved login on the Passwords page. See: Password Manager - Remember, delete and edit logins and passwords in Firefox.

For #2, you usually can type part of the user name in the field and look for the form history suggestion in a drop-down. Press the down arrow key on the keyboard until the suggestion is highlighted, then press Delete (or Shift+Delete) to clear it from form history.

For #3, you could clear your Google cookies this way:

While viewing a page on the www site (for example, Google search), click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page and signing in.

Did any of those work?

Google might also save this on the server as I can see that with mobile devices that show all Gmail email addresses on all devices. Closing all email addresses should reset the list of active accounts, so check the Google account settings.