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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.

From Firefox 53 onwards, no new legacy add-ons will be accepted on (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

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 WebExtensions if they can. See the "Compatibility Milestones" document for more.

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,