--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -2853,32 +2853,52 @@ GeckoDriver.prototype.setScreenOrientati
   }
 
   if (!win.screen.mozLockOrientation(mozOr)) {
     throw new WebDriverError(`Unable to set screen orientation: ${or}`);
   }
 };
 
 /**
- * Maximizes the user agent window as if the user pressed the maximise
- * button.
+ * Synchronously maximizes the user agent window as if the user pressed
+ * the maximize button, or restores it if it is already maximized.
+ *
+ * Not supported on Fennec.
+ *
+ * @return {Map.<string, number>}
+ *     Window rect.
  *
  * @throws {UnsupportedOperationError}
  *     Not available for current application.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.maximizeWindow = function (cmd, resp) {
+GeckoDriver.prototype.maximizeWindow = function* (cmd, resp) {
   assert.firefox();
   const win = assert.window(this.getCurrentWindow());
   assert.noUserPrompt(this.dialog);
 
-  win.maximize()
+  yield new Promise(resolve => {
+    win.addEventListener("resize", resolve, {once: true});
+
+    if (win.windowState == win.STATE_MAXIMIZED) {
+      win.restore();
+    } else {
+      win.maximize();
+    }
+  });
+
+  return {
+    x: win.screenX,
+    y: win.screenY,
+    width: win.outerWidth,
+    height: win.outerHeight,
+  };
 };
 
 /**
  * Dismisses a currently displayed tab modal, or returns no such alert if
  * no modal is displayed.
  */
 GeckoDriver.prototype.dismissDialog = function (cmd, resp) {
   assert.window(this.getCurrentWindow());

new file mode 100644
--- /dev/null
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_window_maximize.py
@@ -0,0 +1,90 @@
+# 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/.
+
+from marionette_driver.errors import InvalidArgumentException
+
+from marionette_harness import MarionetteTestCase
+
+
+class TestWindowMaximize(MarionetteTestCase):
+
+    def setUp(self):
+        MarionetteTestCase.setUp(self)
+        self.max = self.marionette.execute_script("""
+            return {
+              width: window.screen.availWidth,
+              height: window.screen.availHeight,
+            }""", sandbox=None)
+
+        # ensure window is not maximized
+        self.marionette.set_window_size(
+            self.max["width"] - 100, self.max["height"] - 100)
+        actual = self.marionette.window_size
+        self.assertNotEqual(actual["width"], self.max["width"])
+        self.assertNotEqual(actual["height"], self.max["height"])
+
+        self.original_size = actual
+
+    def tearDown(self):
+        self.marionette.set_window_size(
+            self.original_size["width"], self.original_size["height"])
+
+    def assert_window_maximized(self, actual, delta=None):
+        if self.marionette.session_capabilities["platformName"] == "windows_nt":
+            delta = 16
+        else:
+            delta = 8
+
+        self.assertAlmostEqual(
+            actual["width"], self.max["width"],
+            delta=delta,
+            msg="Window width is not within {} px of availWidth: "
+                "current width {} and max width {}"
+                .format(delta, actual["width"], self.max["width"]))
+        self.assertAlmostEqual(
+            actual["height"], self.max["height"],
+            delta=delta,
+            msg="Window height is not within {} px of availHeight, "
+                "current height {} and max height {}"
+                .format(delta, actual["height"], self.max["height"]))
+
+    def assert_window_restored(self, actual):
+        self.assertEqual(self.original_size["width"], actual["width"])
+        self.assertEqual(self.original_size["height"], actual["height"])
+
+    def assert_window_rect(self, rect):
+        self.assertIn("width", rect)
+        self.assertIn("height", rect)
+        self.assertIn("x", rect)
+        self.assertIn("y", rect)
+        self.assertIsInstance(rect["width"], int)
+        self.assertIsInstance(rect["height"], int)
+        self.assertIsInstance(rect["x"], int)
+        self.assertIsInstance(rect["y"], int)
+
+    def test_maximize(self):
+        rect = self.marionette.maximize_window()
+        self.assert_window_rect(rect)
+        size = self.marionette.window_size
+        self.assertEqual(size, rect)
+        self.assert_window_maximized(size)
+
+    def test_maximize_twice_restores(self):
+        maximized = self.marionette.maximize_window()
+        self.assert_window_maximized(maximized)
+
+        restored = self.marionette.maximize_window()
+        self.assert_window_restored(restored)
+
+    def test_stress(self):
+        maximized = False
+
+        for i in range(1, 25):
+            expect_maximized = bool(i % 2)
+
+            rect = self.marionette.maximize_window()
+            if expect_maximized:
+                self.assert_window_maximized(rect)
+            else:
+                self.assert_window_restored(rect)

--- a/testing/marionette/harness/marionette_harness/tests/unit/unit-tests.ini
+++ b/testing/marionette/harness/marionette_harness/tests/unit/unit-tests.ini
@@ -69,16 +69,18 @@ skip-if = appname == 'fennec'
 [test_window_handles_chrome.py]
 skip-if = appname == 'fennec'
 [test_window_handles_content.py]
 [test_window_close_chrome.py]
 skip-if = appname == 'fennec'
 [test_window_close_content.py]
 [test_window_rect.py]
 skip-if = appname == 'fennec'
+[test_window_maximize.py]
+skip-if = appname == 'fennec'
 [test_window_status_content.py]
 [test_window_status_chrome.py]
 
 [test_screenshot.py]
 [test_cookies.py]
 [test_title.py]
 [test_title_chrome.py]
 skip-if = appname == 'fennec'