I am already making an extension for it since a few weeks 
See the progress here: Firebase Extension by arthuro555 · Pull Request #1694 · 4ian/GDevelop · GitHub
I also already have a beginning of documentation here: Firebase [GDevelop wiki] and a beta version here: https://arthuro555-data.s3.eu-de.cloud-object-storage.appdomain.cloud/GDevelop%205%205.0.0-beta101%20firebase.exe
This is a portable build, it won’t override your GDevelop installation.