Override web Firebase cloud messaging background notification with custom behavior

Apparently “notification” fields in your HTTP send request take precedence over any values specified in the service worker (source)

Try encapsulating the notification field in your request with a data field so that

"notification": {
  "title": "hello",
  "body": "world"
}

Becomes:

"data": {
  "notification": {
    "title": "hello",
    "body": "world"
  }
}

If that doesn’t work for you, there are some other workarounds you can try here.