diff --git a/firefox/60-dns_over_https.yml b/firefox/60-dns_over_https.yml
index 309c6534..bc033240 100644
--- a/firefox/60-dns_over_https.yml
+++ b/firefox/60-dns_over_https.yml
@@ -20,6 +20,11 @@ dns_over_https:  # DNS over HTTPS
   custom_dns_url:
     description: Custom DNS URL
     type: web_address
+    validators:
+      - jinja: |-
+          {{ _.custom_dns_url.startswith("http://") }}
+        return_type: boolean
+        description: must starts with 'https://' only
     disabled:
       jinja: |-
         {{ _.provider is propertyerror or _.provider != 'Custom' }}