--- a/testing/marionette/harness/marionette_harness/tests/unit/test_window_rect.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_window_rect.py
@@ -125,16 +125,19 @@ class TestSize(MarionetteTestCase):
             start_size["height"] -= 1
         self.marionette.set_window_size(start_size["width"], start_size["height"])
 
         self.original_size = self.marionette.window_size
 
     def tearDown(self):
         self.marionette.set_window_size(
             self.original_size["width"], self.original_size["height"])
+        is_fullscreen = self.marionette.execute_script("return document.fullscreenElement;", sandbox=None)
+        if is_fullscreen:
+            self.marionette.fullscreen()
         super(MarionetteTestCase, self).tearDown()
 
     def test_get_types(self):
         size = self.marionette.window_size
         self.assertIn("width", size)
         self.assertIn("height", size)
         self.assertIsInstance(size["width"], int)
         self.assertIsInstance(size["height"], int)