browser.captivePortal.onStateChanged.addListener(callback)
browser.captivePortal.onStateChanged.removeListener(listener)
browser.captivePortal.onStateChanged.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 is called when this event occurs. The function is passed the following arguments:

details

string The captive portal state, being one of unknown, not_captive, unlocked_portal, or locked_portal.

Handle a change in captive portal status:

function handlePortalStatus(portalstatusInfo) {
  console.log(`The portal status is now: ${portalstatusInfo.details}`);
}

browser.captivePortal.onStateChanged.addListener(handlePortalStatus);