How can I get the current activity while using Firebase Cloud Messaging in Unity?

This should behave as expected if you change the class back to "com.unity3d.player.UnityPlayer".

The issue is that static methods behave a little differently in Java than standard methods. If you see the Java documentation for the JNI.

The method ID must be derived from clazz, not from one of its superclasses.

Note that this would be contrary to what you may infer from the standard Java documentation (ex: Are static methods inherited in Java?).

To get current unity activity with Firebase Cloud Messaging plugin installed you need to run this code:

var unityPlayer = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
var activity = unityPlayer.GetStatic<AndroidJavaObject>("currentActivity");
// Get current activity name, in my case it returns "com.google.firebase.MessagingUnityPlayerActivity"
Debug.Log(activity.Call<AndroidJavaObject>("getClass").Call<string>("getCanonicalName"));

My android manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" package="com.unity3d.player" android:versionCode="1" android:versionName="1.0">
  <application android:label="@string/app_name" android:icon="@mipmap/app_icon">
    <!-- The MessagingUnityPlayerActivity is a class that extends
         UnityPlayerActivity to work around a known issue when receiving
         notification data payloads in the background. -->
    <activity android:name="com.google.firebase.MessagingUnityPlayerActivity" android:screenOrientation="fullSensor" android:launchMode="singleTask" android:configChanges="mcc|mnc|locale|touchscreen|keyboard|keyboardHidden|navigation|orientation|screenLayout|uiMode|screenSize|smallestScreenSize|fontScale|layoutDirection|density" android:hardwareAccelerated="false">
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
      <meta-data android:name="unityplayer.UnityActivity" android:value="true" />
      <meta-data android:name="android.notch_support" android:value="true" />
    </activity>
    <service android:name="com.google.firebase.messaging.MessageForwardingService" android:exported="false" />
    <meta-data android:name="unity.splash-mode" android:value="0" />
    <meta-data android:name="unity.splash-enable" android:value="True" />
    <meta-data android:name="notch.config" android:value="portrait|landscape" />
    <meta-data android:name="unity.build-id" android:value="" />
  </application>
  <uses-feature android:glEsVersion="0x00030000" />
  <uses-feature android:name="android.hardware.vulkan.version" android:required="false" />
  <uses-feature android:name="android.hardware.touchscreen" android:required="false" />
  <uses-feature android:name="android.hardware.touchscreen.multitouch" android:required="false" />
  <uses-feature android:name="android.hardware.touchscreen.multitouch.distinct" android:required="false" />
</manifest>