diff --git a/layout/tools/reftest/reftest-preferences.js b/layout/tools/reftest/reftest-preferences.js
index 59efb4f7c30..bc1367953fa 100644
--- a/layout/tools/reftest/reftest-preferences.js
+++ b/layout/tools/reftest/reftest-preferences.js
@@ -107,6 +107,7 @@ user_pref("browser.tabs.remote.autostart.1", false);
 user_pref("browser.tabs.remote.autostart.2", false);
 
 user_pref("startup.homepage_welcome_url", "");
+user_pref("startup.homepage_welcome_url.additional", "");
 user_pref("startup.homepage_override_url", "");
 
 user_pref("media.gmp-manager.url.override", "http://localhost/dummy-gmp-manager.xml");