If you scroll down further on the Privacy & Security panel, hopefully you'll find a new section headed Forms & Autofill with that setting. Does it appear on yours?

It is there now, thanks. Took four restarts before it reappeared.

Thank you for your persistence. Address autofill, like Firefox Screenshots, is a bundled official add-on rather than a built-in feature. Possibly there was some issue getting the add-on up and running with Firefox 66 on the first few tries, but hopefully it will be fine going forward.