This is an archived page. It's not actively maintained.



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.



Functions for working with objects.



merge(source, arguments)

Merges all of the properties of all arguments into the first argument. If two or more argument objects have properties with the same name, the property is overwritten with precedence from right to left, implying that properties of the object on the left are overwritten by a same named property of an object on the right. Properties are merged with descriptors onto the source object.

Any argument given with "falsy" values (null, undefined) in case of objects are skipped.

let { merge } = require("sdk/util/object");
var a = { jetpacks: "are yes", foo: 10 }
var b = merge(a, { foo: 5, bar: 6 }, { foo: 50, location: "SF" });

b === a    // true
b.jetpacks // "are yes"      // 50      // 6
b.location // "SF"

// Merge also translates property descriptors
var c = { "type": "addon" };
var d = {};
Object.defineProperty(d, "name", {
  value: "jetpacks",
  configurable: false
merge(c, d);
var props = Object.getOwnPropertyDescriptor(c, "name");
console.log(props.configurable); // true

source : object
The object that other properties are merged into.

arguments : object
n amount of additional objects that are merged into source object.


object : The source object.


Returns an object that inherits from the first argument and contains all of the properties from all following arguments, with precedence from right to left.

extend(source1, source2, source3) is the equivalent of merge(Object.create(source1), source2, source3).

let { extend } = require("sdk/util/object");
var a = { alpha: "a" };
var b = { beta: "b" };
var g = { gamma: "g", alpha: null };
var x = extend(a, b, g);

console.log(a); // { alpha: "a" }
console.log(b); // { beta: "b" }
console.log(g); // { gamma: "g", alpha: null }
console.log(x); // { alpha: null, beta: "b", gamma: "g" }

arguments : object
n arguments that get merged into a new object.


object : The new, merged object.