Hi Pedro.
Firebase cannot send a message. But, the boards can be streaming the database: this means that whenever there’s a change in the database, the boards receive that change on the board. Then, you can add conditions, that depending on the value, turn on or off actuators.
 
There’s a unit in the Firebase ebook about streaming database. After understanding how it works, you can adapt the example to your project.
 
I hope this helps.
Regards, 
Sara