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

addedTabId

integer. ID of the replacement tab.

removedTabId

integer. ID of the tab that was replaced.

Listen for replacement events, and log the associated info:

function handleReplaced(addedTabId, removedTabId) {
  console.log(`New tab: ${addedTabId}`);
  console.log(`Old tab: ${removedTabId}`);
}

browser.tabs.onReplaced.addListener(handleReplaced);