--- a/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
@@ -16,18 +16,27 @@ class TestCapabilities(MarionetteTestCas
         with self.marionette.using_context("chrome"):
             self.appinfo = self.marionette.execute_script("""
                 return {
                   name: Services.appinfo.name,
                   version: Services.appinfo.version,
                   processID: Services.appinfo.processID,
                 }
                 """)
-            self.os_name = self.marionette.execute_script(
-                "return Services.sysinfo.getProperty('name')").lower()
+            self.os_name = self.marionette.execute_script("""
+                let name = Services.sysinfo.getProperty("name");
+                switch (name) {
+                  case "Windows_NT":
+                    return "windows";
+                  case "Darwin":
+                    return "mac";
+                  default:
+                    return name.toLowerCase();
+                }
+                """)
             self.os_version = self.marionette.execute_script(
                 "return Services.sysinfo.getProperty('version')")
 
     def get_fennec_profile(self):
         profile = self.marionette.instance.runner.device.app_ctx.remote_profile
         if self.caps["moz:profile"].lower() != profile.lower():
             # mozdevice may be using a symlink and readlink is not
             # universally available (missing from sdk 18).

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_key_actions.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_key_actions.py
@@ -16,17 +16,17 @@ from marionette_harness import Marionett
 def inline(doc):
     return "data:text/html;charset=utf-8,{}".format(urllib.quote(doc))
 
 
 class TestKeyActions(WindowManagerMixin, MarionetteTestCase):
 
     def setUp(self):
         super(TestKeyActions, self).setUp()
-        if self.marionette.session_capabilities["platformName"] == "darwin":
+        if self.marionette.session_capabilities["platformName"] == "mac":
             self.mod_key = Keys.META
         else:
             self.mod_key = Keys.CONTROL
         test_html = self.marionette.absolute_url("keyboard.html")
         self.marionette.navigate(test_html)
         self.reporter_element = self.marionette.find_element(By.ID, "keyReporter")
         self.reporter_element.click()
         self.key_action = Actions(self.marionette)

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_legacy_mouse_action.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_legacy_mouse_action.py
@@ -17,17 +17,17 @@ def inline(doc):
     return "data:text/html;charset=utf-8,{}".format(urllib.quote(doc))
 
 
 class BaseLegacyMouseAction(MarionetteTestCase):
 
     def setUp(self):
         super(BaseLegacyMouseAction, self).setUp()
 
-        if self.marionette.session_capabilities["platformName"] == "darwin":
+        if self.marionette.session_capabilities["platformName"] == "mac":
             self.mod_key = Keys.META
         else:
             self.mod_key = Keys.CONTROL
 
         self.action = Actions(self.marionette)
 
 
 class TestLegacyMouseAction(BaseLegacyMouseAction):

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_mouse_action.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_mouse_action.py
@@ -60,17 +60,17 @@ class Actions(object):
         return self
 
 
 class BaseMouseAction(MarionetteTestCase):
 
     def setUp(self):
         super(BaseMouseAction, self).setUp()
 
-        if self.marionette.session_capabilities["platformName"] == "darwin":
+        if self.marionette.session_capabilities["platformName"] == "mac":
             self.mod_key = Keys.META
         else:
             self.mod_key = Keys.CONTROL
 
         self.action = Actions(self.marionette)
 
     def tearDown(self):
         super(BaseMouseAction, self).tearDown()

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_navigation.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_navigation.py
@@ -45,17 +45,17 @@ class BaseNavigationTestCase(WindowManag
         self.test_page_file_url = "file:///{}".format(file_path)
         self.test_page_frameset = self.marionette.absolute_url("frameset.html")
         self.test_page_insecure = self.fixtures.where_is("test.html", on="https")
         self.test_page_not_remote = "about:robots"
         self.test_page_push_state = self.marionette.absolute_url("navigation_pushstate.html")
         self.test_page_remote = self.marionette.absolute_url("test.html")
         self.test_page_slow_resource = self.marionette.absolute_url("slow_resource.html")
 
-        if self.marionette.session_capabilities["platformName"] == "darwin":
+        if self.marionette.session_capabilities["platformName"] == "mac":
             self.mod_key = Keys.META
         else:
             self.mod_key = Keys.CONTROL
 
         def open_with_link():
             link = self.marionette.find_element(By.ID, "new-blank-tab")
             link.click()
 

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_switch_window_content.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_switch_window_content.py
@@ -10,17 +10,17 @@ from marionette_driver.keys import Keys
 from marionette_harness import MarionetteTestCase, skip_if_mobile, WindowManagerMixin
 
 
 class TestSwitchToWindowContent(WindowManagerMixin, MarionetteTestCase):
 
     def setUp(self):
         super(TestSwitchToWindowContent, self).setUp()
 
-        if self.marionette.session_capabilities["platformName"] == "darwin":
+        if self.marionette.session_capabilities["platformName"] == "mac":
             self.mod_key = Keys.META
         else:
             self.mod_key = Keys.CONTROL
 
         self.empty_page = self.marionette.absolute_url("empty.html")
         self.test_page = self.marionette.absolute_url("windowHandles.html")
 
         self.selected_tab_index = self.get_selected_tab_index()

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_typing.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_typing.py
@@ -17,17 +17,17 @@ def inline(doc):
     return "data:text/html;charset=utf-8,{}".format(urllib.quote(doc))
 
 
 class TypingTestCase(MarionetteTestCase):
 
     def setUp(self):
         super(TypingTestCase, self).setUp()
 
-        if self.marionette.session_capabilities["platformName"] == "darwin":
+        if self.marionette.session_capabilities["platformName"] == "mac":
             self.mod_key = Keys.META
         else:
             self.mod_key = Keys.CONTROL
 
 
 class TestTypingChrome(TypingTestCase):
 
     def setUp(self):

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_window_maximize.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_window_maximize.py
@@ -26,17 +26,17 @@ class TestWindowMaximize(MarionetteTestC
 
         self.original_size = actual
 
     def tearDown(self):
         self.marionette.set_window_rect(
             width=self.original_size["width"], height=self.original_size["height"])
 
     def assert_window_maximized(self, actual, delta=None):
-        if self.marionette.session_capabilities["platformName"] == "windows_nt":
+        if self.marionette.session_capabilities["platformName"] == "windows":
             delta = 16
         else:
             delta = 22
 
         self.assertGreaterEqual(
             actual["width"], self.max["width"] - delta,
             msg="Window width is not within {delta} px of availWidth: "
                 "current width {current} should be greater than or equal to max width {max}"

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_window_rect.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_window_rect.py
@@ -181,23 +181,23 @@ class TestWindowRect(MarionetteTestCase)
         elif os == "linux":
             # certain WMs prohibit windows from being moved off-screen
             self.assertLessEqual(new_position["x"], 0)
             self.assertLessEqual(new_position["y"], 0)
 
         # On macOS, windows can only be moved off the screen on the
         # horizontal axis.  The system menu bar also blocks windows from
         # being moved to (0,0).
-        elif os == "darwin":
+        elif os == "mac":
             self.assertEqual(-8, new_position["x"])
             self.assertEqual(23, new_position["y"])
 
         # It turns out that Windows is the only platform on which the
         # window can be reliably positioned off-screen.
-        elif os == "windows_nt":
+        elif os == "windows":
             self.assertEqual(-8, new_position["x"])
             self.assertEqual(-8, new_position["y"])
 
     def test_resize_larger_than_screen(self):
         new_size = self.marionette.set_window_rect(
             width=self.max["width"] * 2, height=self.max["height"] * 2)
         actual_size = self.marionette.window_rect
 

--- a/testing/marionette/puppeteer/firefox/firefox_puppeteer/api/keys.py
+++ b/testing/marionette/puppeteer/firefox/firefox_puppeteer/api/keys.py
@@ -3,17 +3,19 @@
 # You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import absolute_import
 
 import marionette_driver
 
 
 class Keys(marionette_driver.keys.Keys):
-    """Proxy to marionette's keys with an "accel" provided for convenience
-    testing across platforms."""
+    """
+    Proxy to Marionette's keys with an "accel" provided for convenience
+    testing across platforms.
+    """
 
     def __init__(self, marionette):
-        self.isDarwin = marionette.session_capabilities['platformName'] == 'darwin'
+        self.is_mac = marionette.session_capabilities["platformName"] == "mac"
 
     @property
     def ACCEL(self):
-        return self.META if self.isDarwin else self.CONTROL
+        return self.META if self.is_mac else self.CONTROL

--- a/testing/marionette/puppeteer/firefox/firefox_puppeteer/ui/browser/window.py
+++ b/testing/marionette/puppeteer/firefox/firefox_puppeteer/ui/browser/window.py
@@ -241,17 +241,17 @@ class BrowserWindow(BaseWindow):
         """
         def callback(win):
             # Prepare action which triggers the opening of the browser window
             if callable(trigger):
                 trigger(win)
             elif trigger == 'menu':
                 self.menubar.select_by_id('tools-menu', 'menu_pageInfo')
             elif trigger == 'shortcut':
-                if win.marionette.session_capabilities['platformName'] == 'windows_nt':
+                if win.marionette.session_capabilities['platformName'] == 'windows':
                     raise ValueError('Page info shortcut not available on Windows.')
                 win.send_shortcut(win.localize_entity('pageInfoCmd.commandkey'),
                                   accel=True)
             elif trigger == 'context_menu':
                 # TODO: Add once we can do right clicks
                 pass
             else:
                 raise ValueError('Unknown opening method: "%s"' % trigger)

--- a/testing/marionette/puppeteer/firefox/firefox_puppeteer/ui/windows.py
+++ b/testing/marionette/puppeteer/firefox/firefox_puppeteer/ui/windows.py
@@ -389,17 +389,17 @@ class BaseWindow(BaseLib):
 
         :param shift: Optional, If `True`, the `Shift` modifier key is pressed.
          Defaults to `False`.
         """
 
         platform = self.marionette.session_capabilities['platformName']
 
         keymap = {
-            'accel': Keys.META if platform == 'darwin' else Keys.CONTROL,
+            'accel': Keys.META if platform == "mac" else Keys.CONTROL,
             'alt': Keys.ALT,
             'cmd': Keys.COMMAND,
             'ctrl': Keys.CONTROL,
             'meta': Keys.META,
             'shift': Keys.SHIFT,
         }
 
         # Append all to press modifier keys