private Light pointLight; // The light component of the firefly

private float minLuminosity = 0; // min intensity
private float maxLuminosity = 1; // max intensity

private float luminositySteps = 0.005f; // factor when increasing / decreasing

private float shineDuration = 3; // wait 3 seconds when faded in

private void Start()
{
    pointLight = GetComponent<Light>();
    pointLight.intensity = Random.Range(minLuminosity, maxLuminosity); // start with a random intensity
    StartCoroutine(ChangeIntensity()); // start the process
}

private IEnumerator ChangeIntensity()
{
    pointLight.intensity += luminositySteps; // increase the firefly intensity / fade in
    yield return new WaitWhile(() => pointLight.intensity >= maxLuminosity); // wait for the maximum intensity

    yield return new WaitForSeconds(shineDuration); // wait 3 seconds

    pointLight.intensity -= luminositySteps;
    yield return new WaitWhile(() => pointLight.intensity <= maxLuminosity); // wait for the minimum intensity

    StartCoroutine(ChangeIntensity()); // do it again
}