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.


An implementation of the node.js child_process API.

This module enables you to execute a child program in a new process. Since it emulates the node.js child_process API, it's not documented separately here. However, there are a few differences to be aware of:

  • you need to require() the module using require("sdk/system/child_process")
  • fork() is not supported
  • gid and uid are not supported
  • in node.js, spawn() and exec() inherit the environment variables from the parent process, by default. The SDK versions don't: so when you specify a command you must pass in a complete path to the command or use the env option to set up the child process environment.
  • child.stdin has no write() method (see example below for writing to child process stdin)


Adaption of node's documentation for spawn():

var child_process = require("sdk/system/child_process");

var ls = child_process.spawn('/bin/ls', ['-lh', '/usr']);

ls.stdout.on('data', function (data) {
  console.log('stdout: ' + data);

ls.stderr.on('data', function (data) {
  console.log('stderr: ' + data);

ls.on('close', function (code) {
  console.log('child process exited with code ' + code);

Writing to child process' stdin

Because the SDK implementation does not include a write() method for child processes, you must use the "raw" emit event.

const { emit } = require('sdk/event/core');
const { spawn } = require('sdk/system/child_process');

var proc = spawn("/bin/cat");

emit(proc.stdin, 'data', "Hello from Add-on code");
emit(proc.stdin, 'end');

Using child_process in non-jpm extensions

// Import SDK Stuff
const COMMONJS_URI = 'resource://gre/modules/commonjs';
const { require } = Cu.import(COMMONJS_URI + '/toolkit/require.js', {});
var child_process = require('sdk/system/child_process');

// Use it in the same way as in the example above


Document Tags and Contributors

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