MDN’s new design is in Beta! A sneak peek:

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 (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.

TypeListeners can be registered with addTypeListener() and will then receive notification as AddonType are registered and unregistered.

Method Overview

void onTypeAdded(in AddonType type)
void onTypeRemoved(in AddonType type)



Called when an add-on type has been added.

void onTypeAdded(
  in AddonType type
The AddonType that is being added
True if an application restart is necessary for the change to take effect


Called when an add-on type has been removed.

void onTypeRemoved(
  in AddonType type,
The AddonType that has been removed

Document Tags and Contributors

 Contributors to this page: bunnybooboo, Leif-AMO, Sheppy, Mossop
 Last updated by: bunnybooboo,