--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -9,16 +9,17 @@ const {utils: Cu} = Components;
 
 const {WebElementEventTarget} = Cu.import("chrome://marionette/content/dom.js", {});
 Cu.import("chrome://marionette/content/element.js");
 const {
   NoSuchWindowError,
   UnsupportedOperationError,
 } = Cu.import("chrome://marionette/content/error.js", {});
 Cu.import("chrome://marionette/content/frame.js");
+const {WindowState} = Cu.import("chrome://marionette/content/wm.js", {});
 
 this.EXPORTED_SYMBOLS = ["browser", "WindowState"];
 
 /** @namespace */
 this.browser = {};
 
 const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 
@@ -446,53 +447,8 @@ browser.Windows = class extends Map {
     let wref = super.get(id);
     if (!wref) {
       throw new RangeError();
     }
     return wref.get();
   }
 
 };
-
-// TODO(ato): Move this to testing/marionette/wm.js
-// after https://bugzil.la/1311041
-/**
- * Marionette representation of the {@link ChromeWindow} window state.
- *
- * @enum {string}
- */
-this.WindowState = {
-  Maximized: "maximized",
-  Minimized: "minimized",
-  Normal: "normal",
-  Fullscreen: "fullscreen",
-
-  /**
-   * Converts {@link nsIDOMChromeWindow.windowState} to WindowState.
-   *
-   * @param {number} windowState
-   *     Attribute from {@link nsIDOMChromeWindow.windowState}.
-   *
-   * @return {WindowState}
-   *     JSON representation.
-   *
-   * @throws {TypeError}
-   *     If <var>windowState</var> was unknown.
-   */
-  from(windowState) {
-    switch (windowState) {
-      case 1:
-        return WindowState.Maximized;
-
-      case 2:
-        return WindowState.Minimized;
-
-      case 3:
-        return WindowState.Normal;
-
-      case 4:
-        return WindowState.Fullscreen;
-
-      default:
-        throw new TypeError(`Unknown window state: ${windowState}`);
-    }
-  },
-};

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -11,20 +11,17 @@ Cu.import("resource://gre/modules/Log.js
 Cu.import("resource://gre/modules/Preferences.jsm");
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 Cu.import("chrome://marionette/content/accessibility.js");
 Cu.import("chrome://marionette/content/addon.js");
 Cu.import("chrome://marionette/content/assert.js");
 Cu.import("chrome://marionette/content/atom.js");
-const {
-  browser,
-  WindowState,
-} = Cu.import("chrome://marionette/content/browser.js", {});
+const {browser} = Cu.import("chrome://marionette/content/browser.js", {});
 Cu.import("chrome://marionette/content/capture.js");
 Cu.import("chrome://marionette/content/cert.js");
 Cu.import("chrome://marionette/content/cookie.js");
 Cu.import("chrome://marionette/content/element.js");
 const {
   ElementNotInteractableError,
   InsecureCertificateError,
   InvalidArgumentError,
@@ -46,16 +43,17 @@ Cu.import("chrome://marionette/content/l
 Cu.import("chrome://marionette/content/modal.js");
 Cu.import("chrome://marionette/content/proxy.js");
 Cu.import("chrome://marionette/content/reftest.js");
 Cu.import("chrome://marionette/content/session.js");
 const {
   PollPromise,
   TimedPromise,
 } = Cu.import("chrome://marionette/content/sync.js", {});
+const {WindowState} = Cu.import("chrome://marionette/content/wm.js", {});
 
 Cu.importGlobalProperties(["URL"]);
 
 this.EXPORTED_SYMBOLS = ["GeckoDriver", "Context"];
 
 const APP_ID_FIREFOX = "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
 
 const FRAME_SCRIPT = "chrome://marionette/content/listener.js";

--- a/testing/marionette/jar.mn
+++ b/testing/marionette/jar.mn
@@ -32,16 +32,17 @@ marionette.jar:
   content/session.js (session.js)
   content/transport.js (transport.js)
   content/packets.js (packets.js)
   content/stream-utils.js (stream-utils.js)
   content/reftest.js (reftest.js)
   content/reftest.xul (reftest.xul)
   content/dom.js (dom.js)
   content/format.js (format.js)
+  content/wm.js (wm.js)
 #ifdef ENABLE_TESTS
   content/test.xul (chrome/test.xul)
   content/test2.xul (chrome/test2.xul)
   content/test_dialog.dtd (chrome/test_dialog.dtd)
   content/test_dialog.properties (chrome/test_dialog.properties)
   content/test_dialog.xul (chrome/test_dialog.xul)
   content/test_nested_iframe.xul (chrome/test_nested_iframe.xul)
   content/test_anonymous_content.xul (chrome/test_anonymous_content.xul)

new file mode 100644
--- /dev/null
+++ b/testing/marionette/wm.js
@@ -0,0 +1,51 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["WindowState"];
+
+/**
+ * Marionette representation of the {@link ChromeWindow} window state.
+ *
+ * @enum {string}
+ */
+const WindowState = {
+  Maximized: "maximized",
+  Minimized: "minimized",
+  Normal: "normal",
+  Fullscreen: "fullscreen",
+
+  /**
+   * Converts {@link nsIDOMChromeWindow.windowState} to WindowState.
+   *
+   * @param {number} windowState
+   *     Attribute from {@link nsIDOMChromeWindow.windowState}.
+   *
+   * @return {WindowState}
+   *     JSON representation.
+   *
+   * @throws {TypeError}
+   *     If <var>windowState</var> was unknown.
+   */
+  from(windowState) {
+    switch (windowState) {
+      case 1:
+        return WindowState.Maximized;
+
+      case 2:
+        return WindowState.Minimized;
+
+      case 3:
+        return WindowState.Normal;
+
+      case 4:
+        return WindowState.Fullscreen;
+
+      default:
+        throw new TypeError(`Unknown window state: ${windowState}`);
+    }
+  },
+};
+this.WindowState = WindowState;