Firebase lets the agent send push notifications to people who use your own app.
In Firebase Console, open Project Settings, then Service Accounts, click Generate New Private Key, and upload the JSON on the agent's Firebase card.
Add your apps: the Android package name, or the iOS app's numeric Apple ID.
Add the FCM topics your app subscribes users to. Each Topic Key must exactly match the name your app passes to subscribeToTopic.
One agent can hold up to 10 Firebase projects. Scheduled pushes go out through the push dispatcher, which runs every hour by default; you can change that (see scheduled tasks).
Nobody receives the push: the topic name doesn't match, devices aren't subscribed to it, or it's the wrong Firebase project.
"invalid JWT signature": the key file is damaged or was replaced. Generate a new key.
More: Firebase integration.