The `unload` module allows modules to register callbacks that are called when they are unloaded. @function 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. @param object {object} An object that defines a destructor method. @param [name] {string} Optional name of the destructor method. Default is `unload`. @function Registers a function to be called when the module is unloaded. @param 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](https://bugzilla.mozilla.org/show_bug.cgi?id=571049).