Polymer app won't load on other browsers like Firefox

Question

Polymer app won't load on other browsers like Firefox

I created an app using Polymer and it works great in Chrome as expected. However, when loading the app in Firefox, for example, I get a blank page.

I have a corresponding import for polyfills, as you can see in my code below. Am I missing something to get it to work in other browsers?

Plunker: http://plnkr.co/edit/gBWxIdOUuizxFH2S5peQ?p=info

<!DOCTYPE html>
<html>
  <head>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <script src="/bower_components/webcomponentsjs/webcomponents.js"></script>

    <link rel="import" href="/bower_components/core-header-panel/core-header-panel.html">
    <link rel="import" href="/bower_components/my-elements/my-appbar.html">
    <link rel="import" href="/bower_components/my-elements/my-content.html">
  </head>

  <body fullbleed vertical layout unresolved>
      <core-header-panel flex>
        <div class="core-header">
          <my-appbar></my-appbar>
        </div>
        <div class="content">
          <my-content></my-content>
        </div>
      </core-header-panel>
  </body>
</html>