runtime.onStartup

All events have three functions:

addListener(listener)

Adds a listener to the calling event.

removeListener(listener)

Stop listening to the calling event. The listener argument is the listener to remove.

hasListener(listener)

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

The only parameter is listener, used for any of the above functions.

listener

A function that is called when this event occurs.

Open https://giphy.com/explore/cat when the browser starts up:

function handleStartup() {
  browser.tabs.create({
    url: "https://giphy.com/explore/cat"
  });
}

browser.runtime.onStartup.addListener(handleStartup);