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.



Access, set and clear environment variables.


var { env } = require('sdk/system/environment');

You can get the value of an environment variable, by accessing the property with the name of the desired variable:

var PATH = env.PATH;

You can check for the existence of an environment variable by checking whether a property with that variable name exists:

console.log('PATH' in env); // true
console.log('FOO' in env);  // false

You can set the value of an environment variable by setting the property:

env.FOO = 'foo';
env.PATH += ':/my/path/'

You can unset an environment variable by deleting the property:

delete env.FOO;


There is no way to enumerate existing environment variables, also env won't have any enumerable properties:

console.log(Object.keys(env)); // []

Environment variable will be unset, show up as non-existing if it's set to null, undefined or ''.

env.FOO = null;
console.log('FOO' in env);  // false
env.BAR = '';
console.log(env.BAR);       // undefined