MDN wants to learn about developers like you:



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.


Register callbacks that are called when a module is unloaded.



ensure(object, name)

Calling ensure() on an object does two things:

  1. It replaces a destructor method with a wrapper method that will never call the destructor more than once.
  2. It ensures that this wrapper method is called when the object's module is unloaded.

Therefore, when you register an object with ensure(), you can call its destructor method yourself, you can let it happen for you, or you can do both.

The destructor will be called with a single argument describing the reason for the unload; see when(). If object does not have the expected destructor method, then an exception is thrown when ensure() is called.


object : object
An object that defines a destructor method.

name : string
Optional name of the destructor method. Default is unload.


Registers a function to be called when the module is unloaded.


callback : function
A function that will be called when the module is unloaded. It is called with a single argument, one of the following strings describing the reason for unload: "uninstall", "disable", "shutdown", "upgrade", or "downgrade". If a reason could not be determined, undefined will be passed instead. Note that if an add-on is unloaded with reason "disable", it will not be notified about "uninstall" while it is disabled. See bug 571049.

Document Tags and Contributors

 Contributors to this page: wbamberg
 Last updated by: wbamberg,