browser.tabs.onAttached.addListener(listener)
browser.tabs.onAttached.removeListener(listener)
browser.tabs.onAttached.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 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:

tabId

integer. ID of the tab that was attached to a new window.

attachInfo

object. ID of the new window, and index of the tab within it. See the attachInfo section for more details.

newWindowId

integer. ID of the new window.

newPosition

integer. Index position that the tab has in the new window.

Listen for attach events, and log the info:

function handleAttached(tabId, attachInfo) {
  console.log(`Tab: ${tabId} attached`);
  console.log(`New window: ${attachInfo.newWindowId}`);
  console.log(`New index: ${attachInfo.newPosition}`);
}

browser.tabs.onAttached.addListener(handleAttached);