要在Android应用程序被杀死时接收Firebase云消息，您可以使用Firebase Cloud Messaging（FCM）的数据消息类型，并在主活动的onCreate方法中注册一个BroadcastReceiver来处理消息。

以下是一个简单的示例，演示如何在应用程序被杀死时接收Firebase云消息：

1. 在您的应用程序级别的build.gradle文件中添加Firebase消息依赖项：

implementation 'com.google.firebase:firebase-messaging:20.3.0'

2. 创建一个名为MyFirebaseMessagingService的类，继承自FirebaseMessagingService。在此类中，重写onMessageReceived方法，以处理接收到的消息。在此方法中，您可以将消息发送到BroadcastReceiver来进行处理：

import android.content.Intent;
import androidx.localbroadcastmanager.content.LocalBroadcastManager;
import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        // 处理接收到的消息
        String message = remoteMessage.getData().get("message");

        // 将消息发送到BroadcastReceiver
        Intent intent = new Intent("FIREBASE_MESSAGE");
        intent.putExtra("message", message);
        LocalBroadcastManager.getInstance(this).sendBroadcast(intent);
    }
}

3. 创建一个名为MyBroadcastReceiver的类，继承自BroadcastReceiver。在此类中，重写onReceive方法，以处理接收到的Firebase消息：

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.widget.Toast;

public class MyBroadcastReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        // 处理接收到的消息
        String message = intent.getStringExtra("message");
        Toast.makeText(context, message, Toast.LENGTH_SHORT).show();
    }
}

4. 在您的主活动的onCreate方法中注册BroadcastReceiver，并在需要时启动Firebase服务：

import android.content.BroadcastReceiver;
import android.content.IntentFilter;
import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.localbroadcastmanager.content.LocalBroadcastManager;
import com.google.firebase.messaging.FirebaseMessaging;

public class MainActivity extends AppCompatActivity {

    private BroadcastReceiver broadcastReceiver;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 注册BroadcastReceiver
        broadcastReceiver = new MyBroadcastReceiver();
        IntentFilter intentFilter = new IntentFilter("FIREBASE_MESSAGE");
        LocalBroadcastManager.getInstance(this).registerReceiver(broadcastReceiver, intentFilter);

        // 启动Firebase服务
        FirebaseMessaging.getInstance().subscribeToTopic("your_topic");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        // 取消注册BroadcastReceiver
        LocalBroadcastManager.getInstance(this).unregisterReceiver(broadcastReceiver);
    }
}

请确保在AndroidManifest.xml文件中声明MyFirebaseMessagingService类：

通过这种方式，即使您的应用程序被杀死，您仍然可以接收到Firebase云消息，并通过BroadcastReceiver进行处理。