browser.windows.onRemoved.addListener(listener)
browser.windows.onRemoved.removeListener(listener)
browser.windows.onRemoved.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)

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

callback

Function that will be called when this event occurs. The function will be passed the following arguments:

windowId

integer. ID of the window that was closed.

Log the IDs of windows as they are removed.

browser.windows.onRemoved.addListener((windowId) => {
  console.log(`Closed window: ${windowId}`);
});