We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

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 instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

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

Document Tags and Contributors

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