title: How to use web browser download api?
body: <p>In this article <a href="https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/downloads/download" rel="nofollow noreferrer">downloads on MDN</a>, it says we can use downloads api to donwload a file, but code in the example of that article</p>
  <pre><code>var downloading = browser.downloads.download({
    url : downloadUrl,
    filename : 'my-image-again.png',
    conflictAction : 'uniquify'
  }); 
  
  downloading.then(onStartedDownload, onFailed);
  </code></pre>
  <p>will lead to <code>Uncaught ReferenceError: browser is not defined</code>.
  I ran this code in chrome console pannel. Did I miss something? <br></p>