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

highlightInfo

object. ID(s) of the highlighted tabs, and ID of their window. See the highlightInfo section for more details.

windowId

integer. ID of the window whose tabs changed.

tabIds

array of integer. IDs of the highlighted tabs in the window.

Listen for highlighting events, and log the IDs of highlighted tabs:

function handleHighlighted(highlightInfo) {
  console.log(`Highlighted tabs: ${highlightInfo.tabIds}`);
}

browser.tabs.onHighlighted.addListener(handleHighlighted);