diff --git a/Optional-Hardening.md b/Optional-Hardening.md
index e2ab26c..af3fec3 100644
--- a/Optional-Hardening.md
+++ b/Optional-Hardening.md
@@ -9,6 +9,7 @@ We already:
 // PREF: enable HTTPS-Only Mode
 // Strict HTTPS site navigation in Normal and Private Browsing.
 user_pref("dom.security.https_only_mode", true);
+user_pref("dom.security.https_only_mode_error_page_user_suggestions", true);
 ```
 
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