diff --git a/src/main/Main.coffee b/src/main/Main.coffee
index 2975579a6..8ccc9948b 100644
--- a/src/main/Main.coffee
+++ b/src/main/Main.coffee
@@ -3,6 +3,9 @@ Main =
     # XXX Work around Pale Moon / old Firefox + GM 1.15 bug where script runs in iframe with wrong window.location.
     return if d.body and not $ 'title', d.head
 
+    # Don't run inside ad iframes.
+    return if window.frameElement and window.frameElement.src is ''
+
     # XXX dwb userscripts extension reloads scripts run at document-start when replaceState/pushState is called.
     return if window['<%= meta.name %> antidup']
     window['<%= meta.name %> antidup'] = true