Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

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.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

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 use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

Add-on types hold useful information about each type of add-on that may be installed. They are mostly used to help the UI know how to display the different types of add-on.


Attribute Type Description
id string The unique ID for the type.
name string The localized name for the type.
uiPriority integer

A number used to determine the order of types displayed in the user interface. The lower the number the higher in the list the type will appear.

Built-in Values:

Value Category
2000 Locale
4000 Extension
5000 Theme
6000 Plugin
viewType integer The type of UI to use to display this type of add-on in the UI. If undefined the type will not appear in the UI.
flags integer Additional behavior flags for the type.