FCM Toolbox is a free Android app developed by Simon Marquis that helps users test and debug Google's Firebase Cloud Messaging (FCM) service. With this app, you can easily send and receive FCM payloads, manage registered devices, and self-host your FCM instances.

The app's user interface is straightforward and easy to navigate, making it a great tool for developers and non-developers alike. You can quickly test your FCM service by sending payloads to your registered devices and receiving responses in real-time. Additionally, you can manage your registered devices and view their FCM instance IDs, making it easy to debug any issues that may arise.

FCM Toolbox also offers self-hosting capabilities, allowing you to host your own FCM instances and test them without having to rely on Google's infrastructure. However, it's important to note that on the public toolbox versions, users share the same FCM and FRD instances, so it's important to be responsible.