downloads.removeFile()

let removing = browser.downloads.removeFile(
  downloadId      // integer
)

function onRemoved() {
  console.log(`Removed item`);
}
function onError(error) {
  console.log(`Error: ${error}`);
}
function remove(downloadItems) {
  if (downloadItems.length > 0) {
    let removing = browser.downloads.removeFile(downloadItems[0].id);
    removing.then(onRemoved, onError);
  }
}
let searching = browser.downloads.search({
  limit: 1,
  orderBy: ["-startTime"],
});
searching.then(remove, onError);