force web download

forceDownload.js

page.evaluate( () => {

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

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

  var downloadUrl = "https://example.org/image.png";

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

  downloading.then(onStartedDownload, onFailed);

})