Firefox ReferenceError：未定义浏览器，API browser.downloads.download

Question

有人可以帮助我理解为什么它在FF中不起作用以及我需要做些什么才能使其起作用吗？

index.html

<meta charset="utf-8">
<script type="application/javascript"
src="https://unpkg.com/webextension-polyfill@0.2.1/dist/browser-polyfill.min.js">
</script>
<script>
function onStartedDownload(id) {
  console.log(`Started downloading: ${id}`);
}

function onFailed(error) {
  console.log(`Download failed: ${error}`);
}

var downloadUrl = "https://sample-videos.com/img/Sample-jpg-image-50kb.jpg";

var downloading = browser.downloads.download({
  url : downloadUrl,
  filename : 'my-image-again.png',
  conflictAction : 'uniquify',
  saveAs : 'true'
});

downloading.then(onStartedDownload, onFailed);
</script>

我只使用一个文件index.html

downloads API的download（）函数根据文件的URL和其他可选首选项下载文件。

代码段

function onStartedDownload(id) {
  console.log(`Started downloading: ${id}`);
}

function onFailed(error) {
  console.log(`Download failed: ${error}`);
}

var downloadUrl = "https://sample-videos.com/img/Sample-jpg-image-50kb.jpg";

var downloading = browser.downloads.download({
  url : downloadUrl,
  filename : 'my-image-again.png',
  conflictAction : 'uniquify',
  saveAs : 'true'
});

downloading.then(onStartedDownload, onFailed);

<meta charset="utf-8">