symfony: The "firebase" scheme is not supported.

I’ve got a working POC extending Notification and Recipient classes:

class FirebaseAwareRecipient extends Recipient
{
    private string $firebaseToken;

    public function __construct(string $email = '', string $phone = '', string $firebaseToken = '')
    {
        parent::__construct($email, $phone);
        $this->firebaseToken = $firebaseToken;
    }

    public function getFirebaseToken(): string
    {
        return $this->firebaseToken;
    }

}

class FirebaseAwareNotification extends Notification implements ChatNotificationInterface
{
    public function asChatMessage(RecipientInterface $recipient, string $transport = null): ?ChatMessage
    {
        $message = ChatMessage::fromNotification($this);
        if('firebase' === $transport) {
            $message->subject($this->getContent())
            ->options(new AndroidNotification(
                $recipient->getFirebaseToken(),
                ['title' => $this->getSubject()]
            ));
        }
        return $message;    
    }
}

$notification = (new FirebaseAwareNotification('New Invoice'))
    ->content('You got a new invoice for 15 EUR.')
    ->importance(Notification::IMPORTANCE_HIGH);
$this->notifier->send($notification, new FirebaseAwareRecipient('user@example.com', '', 'FIREBASE-TOKEN-HERE')));

Thanks a lot for the answer. Thanks to your help, I fixed all the problems and now the notifications work perfectly. Hopefully someone will work on improving the Notifier documentation.

I am sorry for my last comment. I had a deeper look at the firebase-notifier and its poor documentation. This code (https://github.com/symfony/firebase-notifier/blob/5.4/FirebaseTransport.php#L48:L51) is the key. You have to use firebase with ChatMessage instances and recording to the notifier documentation please try the following (as I don’t use firebase):

use Symfony\Component\Notifier\Bridge\Firebase\Notification\AndroidNotification;
use Symfony\Component\Notifier\Message\ChatMessage;
use Symfony\Component\Notifier\ChatterInterface;

public function __invoke(ChatterInterface $chatter)
{
    // you can also use WebNotification or IOSNotification with their special options
    // the second array is used for payload options
    $options = new AndroidNotification('to', []);

    $message = (new ChatMessage('Hello world!'))
        ->transport('firebase')
        ->options($options);

    $sentMessage = $chatter->send($message);
}

Edit: And don’t forget to move firebase: '%env(FIREBASE_DSN)%' from texter_transports to chatter_transports in notifier.yaml.