browser.tabs.onDetached.addListener(listener)
browser.tabs.onDetached.removeListener(listener)
browser.tabs.onDetached.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 detached.

detachInfo

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

oldWindowId

integer. ID of the previous window.

oldPosition

integer. Index position that the tab had in the old window.

Listen for detach events, and log the info:

function handleDetached(tabId, detachInfo) {
  console.log(`Tab: ${tabId} moved`);
  console.log(`Old window: ${detachInfo.oldWindowId}`);
  console.log(`Old index: ${detachInfo.oldPosition}`);
}

browser.tabs.onDetached.addListener(handleDetached);