That's unsettling!

Usually the address autofill waits for you to start typing or select from a drop-down, so I don't think that feature is responsible. Not to mention that you turned it off.

If you click in the field and press the Down arrow key on the keyboard, is it listed as a suggestion? Firefox's form history may remember entries, although that wouldn't usually appear on a new website. To remove form history entries, arrow down to highlight the unwanted item in the drop down and press the Delete key (or Shift+Delete on some platforms). To disable form history: Control whether Firefox automatically fills in forms.

To clear cached information from forms, reload the page by passing the cache. For example, press Ctrl+Shift+R or hold the Shift key while clicking the reload button on the toolbar.

If the data persists, perhaps the site knows you through a relationship it has with another site where you have an account.

Alternately, perhaps you have an add-on that fills forms. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything that involves filling logins or forms? Anything you just do not remember installing or why? If in doubt, disable (or remove).

Any progress so far?

Thank you, JScher2000 for answering. It turns out, actually, that the issue wasn't Firefox. I found that the website in question was affiliated with another site where I do, in fact, have my address stored.