below is a failure excerpt from windows 7 (test reports success on windows 10): 18:59:55 INFO - Running tests on Firefox 52.0a1/Gecko 52.0a1 (Build 20161020153615) ({ec8030f7-c20a-464f-9b0e-13a3a9e97384}) under winnt/x86. 18:59:55 INFO - TEST-START | child_process/index.test child_process in an addon 18:59:56 WARNING - TEST-UNEXPECTED-FAIL | child_process/index.test child_process in an addon | no errors - { // [object Object] 18:59:56 INFO - // writable configurable enumerable 18:59:56 INFO - message: "Failed to create process", 18:59:56 INFO - // writable configurable enumerable 18:59:56 INFO - fileName: "resource://gre/modules/subprocess/subprocess_worker_win.js", 18:59:56 INFO - // writable configurable enumerable 18:59:56 INFO - lineNumber: 526, 18:59:56 INFO - // writable configurable enumerable 18:59:56 INFO - column: undefined, 18:59:56 INFO - // writable configurable enumerable 18:59:56 INFO - stack: "spawn@resource://gre/modules/subprocess/subprocess_worker_win.js:526:13 18:59:56 INFO - BaseProcess@resource://gre/modules/subprocess/subprocess_worker_common.js:61:5 18:59:56 INFO - Process@resource://gre/modules/subprocess/subprocess_worker_win.js:321:11 18:59:56 INFO - spawn@resource://gre/modules/subprocess/subprocess_worker_common.js:123:19 18:59:56 INFO - onmessage/<@resource://gre/modules/subprocess/subprocess_worker_common.js:194:27 18:59:56 INFO - onmessage@resource://gre/modules/subprocess/subprocess_worker_common.js:193:3 18:59:56 INFO - EventHandlerNonNull*@resource://gre/modules/subprocess/subprocess_worker_common.js:188:1 18:59:56 INFO - @resource://gre/modules/subprocess/subprocess_worker_win.js:11:1 18:59:56 INFO - ", 18:59:56 INFO - // writable configurable enumerable 18:59:56 INFO - errorCode: undefined, 18:59:56 INFO - "__proto__": { // [object Object] 18:59:56 INFO - } 18:59:56 INFO - } == null 18:59:56 INFO - TEST-INFO | Traceback (most recent call last): 18:59:56 INFO - File "resource://gre/modules/Promise-backend.js", line 816, in this.PromiseWalker.walkerLoop 18:59:56 INFO - this.handlers.shift().process(); 18:59:56 INFO - File "resource://gre/modules/Promise-backend.js", line 940, in Handler.prototype.process 18:59:56 INFO - nextValue = this.onReject.call(undefined, nextValue); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process/subprocess.js", line 139, in subprocess.call/< 18:59:56 INFO - options.done({exitCode: -1}, e); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 66, in initialize/proc<.done 18:59:56 INFO - return handleError(error); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 108, in handleError 18:59:56 INFO - emit(child, 'error', error); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/event/core.js", line 89, in emit 18:59:56 INFO - emitOnObject(target, type, target, ...args); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/event/core.js", line 112, in emitOnObject 18:59:56 INFO - listener.apply(thisArg, args); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 268, in errorHandler 18:59:56 INFO - exitHandler(); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 258, in exitHandler 18:59:56 INFO - callback(error, stdout, stderr); 18:59:56 INFO - File "resource://test-child-process-at-jetpack/child_process/lib/index.js", line 25, in exports["test child_process in an addon"]/< 18:59:56 INFO - assert.equal(err, null, "no errors"); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/test/assert.js", line 123, in equal 18:59:56 INFO - this.fail({ 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/test/assert.js", line 88, in fail 18:59:56 INFO - this._log.fail(message); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/deprecated/unit-test.js", line 104, in fail 18:59:56 INFO - this.console.testMessage(false, false, this.test.name, message); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/test/harness.js", line 541, in testMessage 18:59:56 INFO - this.trace(); 18:59:56 INFO - TEST-PASS | child_process/index.test child_process in an addon | stderr is empty 18:59:56 WARNING - TEST-UNEXPECTED-FAIL | child_process/index.test child_process in an addon | stdout output of `ls -al` finds files - false == true 18:59:56 INFO - TEST-INFO | Traceback (most recent call last): 18:59:56 INFO - File "resource://gre/modules/Promise-backend.js", line 816, in this.PromiseWalker.walkerLoop 18:59:56 INFO - this.handlers.shift().process(); 18:59:56 INFO - File "resource://gre/modules/Promise-backend.js", line 940, in Handler.prototype.process 18:59:56 INFO - nextValue = this.onReject.call(undefined, nextValue); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process/subprocess.js", line 139, in subprocess.call/< 18:59:56 INFO - options.done({exitCode: -1}, e); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 66, in initialize/proc<.done 18:59:56 INFO - return handleError(error); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 108, in handleError 18:59:56 INFO - emit(child, 'error', error); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/event/core.js", line 89, in emit 18:59:56 INFO - emitOnObject(target, type, target, ...args); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/event/core.js", line 112, in emitOnObject 18:59:56 INFO - listener.apply(thisArg, args); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 268, in errorHandler 18:59:56 INFO - exitHandler(); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/system/child_process.js", line 258, in exitHandler 18:59:56 INFO - callback(error, stdout, stderr); 18:59:56 INFO - File "resource://test-child-process-at-jetpack/child_process/lib/index.js", line 27, in exports["test child_process in an addon"]/< 18:59:56 INFO - assert.ok(/extensions\.ini/.test(stdout), "stdout output of `ls -al` finds files"); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/test/assert.js", line 99, in ok 18:59:56 INFO - this.fail({ 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/test/assert.js", line 88, in fail 18:59:56 INFO - this._log.fail(message); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/deprecated/unit-test.js", line 104, in fail 18:59:56 INFO - this.console.testMessage(false, false, this.test.name, message); 18:59:56 INFO - File "resource://gre/modules/commonjs/sdk/test/harness.js", line 541, in testMessage 18:59:56 INFO - this.trace();

this is working in the latest worker/ami configuration