diff --git a/firefox/60-dns_over_https.yml b/firefox/60-dns_over_https.yml
index cd5be39..0c9cc15 100644
--- a/firefox/60-dns_over_https.yml
+++ b/firefox/60-dns_over_https.yml
@@ -23,3 +23,9 @@ dns_over_https:  # DNS over HTTPS
         provider is not custom
         {% endif %}
       description: if "dns_over_https.provider" is not "Custom"
+    validators:
+      - jinja: |
+          {% if dns_over_https.custom_dns_url.startswith('http://') %}
+          only https is allowed
+          {% endif %}
+        description: must starts with 'https://' only