browser.downloads.onCreated.addListener(listener)
browser.downloads.onCreated.removeListener(listener)
browser.downloads.onCreated.hasListener(listener)

Events have three functions:

addListener(callback)

Adds a listener to this event.

removeListener(listener)

Stop listening to this event. The listener argument is the listener to remove.

hasListener(listener)

Check whether a given listener is registered for this event. Returns true if it is listening, false otherwise.

function

A callback function that will be called when this event occurs. This function will be passed the following arguments:

downloadItem

The downloads.DownloadItem object in question.

Log the URL of items as they are downloaded:

function handleCreated(item) {
  console.log(item.url);
}

browser.downloads.onCreated.addListener(handleCreated);