diff --git a/firefox/30-auto.yml b/firefox/30-auto.yml
index 77e252b6..50961391 100644
--- a/firefox/30-auto.yml
+++ b/firefox/30-auto.yml
@@ -5,4 +5,7 @@ version: 1.1
 auto:
   description: Automatic proxy configuration URL
   type: web_address
+  disabled:
+    variable: _.proxy_mode
+    when_not: Automatic proxy configuration URL
 ...