--- a/testing/firefox-ui/harness/firefox_ui_harness/testcases.py
+++ b/testing/firefox-ui/harness/firefox_ui_harness/testcases.py
@@ -264,17 +264,17 @@ class UpdateTestCase(FirefoxTestCase):
                                                              ]),
                                                     message='Final wizard page has been selected.')
 
                 else:
                     raise Exception('Invalid wizard page for downloading an update: {}'.format(
                                     dialog.wizard.selected_panel))
 
             finally:
-                prefs.restore_pref(self.PREF_APP_UPDATE_ALTWINDOWTYPE)
+                self.marionette.clear_pref(self.PREF_APP_UPDATE_ALTWINDOWTYPE)
 
         # The old update wizard dialog has to be handled differently. It's necessary
         # for fallback updates and invalid add-on versions.
         if isinstance(window, UpdateWizardDialog):
             download_via_update_wizard(window)
             return
 
         if window.deck.selected_panel == window.deck.download_and_install: