If you have downloaded some complete firebase project, and as you try to install them, you will find firebase set up does not work.
Or throws error as it tries to connect with Firebase. In that case, I will suggest you do the work dirty way.
- create a new flutter project
- create a new firebase project
- connect both of them together
- then from the downloaded project copy the lib, assets and pubspec.ymal file plugins names
- then do flutter pub get
- you should also copy some other assets like strings.xml and put inside android/app/src/main/res/values/ folder and android/app/src/main/res/row
- if you use sound for notification, you should also copy the sound file in the correct place like in iOS/Runner/
- then run your app