Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
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.