browser.tabs.onActiveChanged.addListener(listener)
browser.tabs.onActiveChanged.removeListener(listener)
browser.tabs.onActiveChanged.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. The ID of the tab that has become active.

selectInfo

object. See the selectInfo section for more details.

windowId

integer. The ID of the window containing the selected tab.