diff --git a/browser/components/extensions/ext-utils.js b/browser/components/extensions/ext-utils.js
index e2e8ea73e723..91db5bdda2a3 100644
--- a/browser/components/extensions/ext-utils.js
+++ b/browser/components/extensions/ext-utils.js
@@ -738,6 +738,11 @@ global.WindowManager = {
       state = "fullscreen";
     }
 
+    let xulWindow = window.QueryInterface(Ci.nsIInterfaceRequestor)
+                          .getInterface(Ci.nsIDocShell)
+                          .treeOwner.QueryInterface(Ci.nsIInterfaceRequestor)
+                          .getInterface(Ci.nsIXULWindow);
+
     let result = {
       id: this.getId(window),
       focused: window.document.hasFocus(),
@@ -748,6 +753,7 @@ global.WindowManager = {
       incognito: PrivateBrowsingUtils.isWindowPrivate(window),
       type: this.windowType(window),
       state,
+      alwaysOnTop: xulWindow.zLevel >= Ci.nsIXULWindow.raisedZ,
     };
 
     if (getInfo && getInfo.populate) {
diff --git a/browser/components/extensions/schemas/windows.json b/browser/components/extensions/schemas/windows.json
index 88a0f0149efa..a733b5da8278 100644
--- a/browser/components/extensions/schemas/windows.json
+++ b/browser/components/extensions/schemas/windows.json
@@ -88,7 +88,6 @@
             "description": "The state of this browser window."
           },
           "alwaysOnTop": {
-            "unsupported": true,
             "type": "boolean",
             "description": "Whether the window is set to be always on top."
           },
diff --git a/browser/components/extensions/test/browser/browser.ini b/browser/components/extensions/test/browser/browser.ini
index 2480a1bab11b..10a1f112af46 100644
--- a/browser/components/extensions/test/browser/browser.ini
+++ b/browser/components/extensions/test/browser/browser.ini
@@ -56,6 +56,7 @@ support-files =
 [browser_ext_windows_create.js]
 tags = fullscreen
 [browser_ext_windows_create_tabId.js]
+[browser_ext_windows.js]
 [browser_ext_windows_update.js]
 tags = fullscreen
 [browser_ext_contentscript_connect.js]
diff --git a/browser/components/extensions/test/browser/browser_ext_windows.js b/browser/components/extensions/test/browser/browser_ext_windows.js
new file mode 100644
index 000000000000..d3dd6ecdbe06
--- /dev/null
+++ b/browser/components/extensions/test/browser/browser_ext_windows.js
@@ -0,0 +1,33 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+"use strict";
+
+add_task(function* () {
+  let raisedWin = Services.ww.openWindow(
+    null, Services.prefs.getCharPref("browser.chromeURL"), "_blank",
+    "chrome,dialog=no,all,alwaysRaised", null);
+
+  yield TestUtils.topicObserved("browser-delayed-startup-finished",
+                                subject => subject == raisedWin);
+
+  let extension = ExtensionTestUtils.loadExtension({
+    background: function() {
+      browser.windows.getAll((wins) => {
+        browser.test.assertEq(wins.length, 2, "Expect two windows");
+
+        browser.test.assertEq(false, wins[0].alwaysOnTop,
+                              "Expect first window not to be always on top");
+        browser.test.assertEq(true, wins[1].alwaysOnTop,
+                              "Expect first window to be always on top");
+
+        browser.test.notifyPass("alwaysOnTop");
+      });
+    },
+  });
+
+  yield extension.startup();
+  yield extension.awaitFinish("alwaysOnTop");
+  yield extension.unload();
+
+  yield BrowserTestUtils.closeWindow(raisedWin);
+});