diff --git a/testing/marionette/browser.js b/testing/marionette/browser.js
index dead62df27a2..924731ae73d9 100644
--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -353,7 +353,11 @@ browser.Context = class {
           subject => subject == win
         );
 
-        win.focus();
+        // Bug 1509380 - Missing focus/activate event when Firefox is not
+        // the top-most application. As such wait for the next tick, and
+        // manually focus the newly opened window.
+        win.setTimeout(() => win.focus(), 0);
+
         await Promise.all([activated, focused, startup]);
 
         // The new window shouldn't get focused. As such set the
diff --git a/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py b/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
index e98eddae91b1..79f702ebc258 100644
--- a/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
+++ b/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
@@ -163,7 +163,10 @@ class WindowManagerMixin(object):
                     let win = window.openDialog(url, null, "chrome,centerscreen");
                     let focused = waitForFocus(win);
 
-                    win.focus();
+                    // Bug 1509380 - Missing focus/activate event when Firefox is not
+                    // the top-most application. As such wait for the next tick, and
+                    // manually focus the newly opened window.
+                    win.setTimeout(() => win.focus(), 0);
                     await focused;
 
                     // The new window shouldn't get focused. As such set the