runtime.onSuspend

Sent to the event page just before it is unloaded. This gives the extension an opportunity to do some cleanup. Note that since the page is unloading, any asynchronous operations started while handling this event are not guaranteed to complete.

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);