Turning autocomplete on for a XUL widget
This article is about XUL widgets and not about HTML form inputs.
First, declare a panel with the "autocomplete" type, like so:
<panel id="popup_autocomplete" type="autocomplete" noautofocus="true" />
Now set the autocompletepopup attribute of your <browser> element to the id of the panel you just declared:
<browser id="my_browser" ... autocompletepopup="popup_autocomplete" />
Finally, make sure that the value of the browser.formfill.enable pref is set to true.