Before going to configure firebase push notification on ios and android platform you need to configure your project on firebase console.
You have to create project and ios and android application on firebase console.
Add bundle identifier of ios and application packages id of Android project.
Please follow all the steps which are mentioned on firebase console for both iOS and Android application.
Download google-service.json file for Android
Download Google-service-info.plist file for iOS
Once every things is done you need to drag and drop the file to App level folder on Android.
For iOS platform follow all the steps which are mentioned on firebase console.
For this you must have enrolled apple developer account.
Register your application with updated bundle id.
Configure push notification and download development and distribution certificates.
Add those certificates to your system keychain with double click on that certificates.
Generate .p12 certificates and add both the certificates on firebase console.
Download GoogleService-Info.plist file.
Drag and Drop that file on ios -> runner folder.
We have mentioned all push notification related links on important links section