ref: FCM API

Reading angular/angularfire#2299 (comment), seems it was working before 7.6.0. Any idea what changed?

Finally got a chance to route back to this. I think I found the cause for this: currently window controller stores a member variable onMessageCallback which gets fed with value from nextOrObserver. Internally, if this nextOrObserver is not a callback function, we do this.onMessageCallback = nextOrObserver.next assuming the next callback is passed directly from the observer object developer provided in

{
  next: (msg) => handleForegroundMessage(msg),
  error: (err) => console.log("err: " + err),
}

However, rxjs actually does some internal operation and converts the observer into a subscriber instance, such instance should be used as a unit as it is (e.x. subscriber.next()). Trying to extract the next callback from it will result in

this.onMessageCallback = function: ƒ (value) {
           if (!this.isStopped) {
               this._next(value);
           }
       }

since next is stripped away from the generated subscriber, this._next nor this.isStopped is undefined.
I have a PR that would resolve this issue.

perfect, I'm coming from angular/angularfire#2497 (comment) and I'm wating for your fix to update my PR