downloads.show()

let showing = browser.downloads.show(
  downloadId             // integer
)

function onShowing(success) {
  console.log(`Showing download item: ${success}`);
}

function onError(error) {
  console.log(`Error opening item: ${error}`);
}

function openDownload(downloadItems) {
  if (downloadItems.length > 0) {
    latestDownloadId = downloadItems[0].id;
    let showing = browser.downloads.show(latestDownloadId);
    showing.then(onShowing, onError);
  }
}

let searching = browser.downloads.search({
  limit: 1,
  orderBy: ["-startTime"],
});

searching.then(openDownload, onError);