diff -r 000000000000 -r 6474c204b198 browser/base/content/test/general/browser_duplicateIDs.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/base/content/test/general/browser_duplicateIDs.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,8 @@
+function test() {
+  var ids = {};
+  Array.forEach(document.querySelectorAll("[id]"), function (node) {
+    var id = node.id;
+    ok(!(id in ids), id + " should be unique");
+    ids[id] = null;
+  });
+}