diff --git a/firefox/60-dns_over_https.yml b/firefox/60-dns_over_https.yml
index 6ce2c7ef..55b37cc6 100644
--- a/firefox/60-dns_over_https.yml
+++ b/firefox/60-dns_over_https.yml
@@ -20,6 +20,12 @@ dns_over_https:  # DNS over HTTPS
   custom_dns_url:
     description: Custom DNS URL
     type: web_address
+    validators:
+      - jinja: |-
+          {% if _.custom_dns_url.startswith('http://') %}
+            only https is allowed
+          {% endif %}
+        description: must starts with 'https://' only
     disabled:
       jinja: |-
         {% if _.provider is propertyerror or _.provider != 'Custom' %}