browser.runtime.onSuspend.addListener(listener)
browser.runtime.onSuspend.removeListener(listener)
browser.runtime.onSuspend.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)

Checks whether a listener is registered for this event. Returns true if it is listening, false otherwise.

callback

Function that will be called when this event occurs.

Listen for suspend events:

function handleSuspend() {
  console.log("Suspending event page");
  // handle cleanup
}

browser.runtime.onSuspend.addListener(handleSuspend);