To turn on autocomplete in the address bar:
Firefox button > Options > Options > Privacy
(or Tools > Options > Privacy)
Under the Location bar section:
Choose Bookmarks, History or Bookmarks and History

To turn on autocomplete in the form fields:
Firefox button > Options > Options > Privacy
(or Tools > Options > Privacy)
Under the History section:
Put a checkmark on Remember search and form history

Okay; all my settings appear to have been correct, but, there being no FF toolbar, how do I "activate" autocomplete after logging in to Firefox?

Which autocomplete are you referring to (forms on websites or the location bar)?

If the menu bar is hidden then press the F10 key or hold down the Alt key to make the menu bar appear.

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible: "View > Toolbars"