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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>buildID — privacy</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var FIXED_FX = "20181001000000";
    var out;
    if (!("buildID" in navigator)) {
      out = "no buildID";
    } else if (navigator.buildID === FIXED_FX) {
      out = "matches Firefox privacy placeholder";
    } else {
      out = "other buildID: " + navigator.buildID;
    }
    document.getElementById("out").textContent = out;
  </script>
</body>
</html>

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buildID — privacy placeholder (Try It)