The upcoming changes to the Apple Push Notification service (APNs) server certificates, including the SHA-2 Root: USERTrust RSA Certification Authority update, will not directly impact developers using Firebase Cloud Messaging (FCM) for push notifications. Firebase, as a third-party provider, will handle the necessary updates to its server certificates.

Apple's Official Statement: From the Apple Developer : https://forums.developer.apple.com/forums/thread/770316

Forums:

This certificate update is only needed for developers who are running their own push servers to directly communicate with APNs. 3rd party push providers will need to handle this certificate update on their end.

Recommendation: No additional action is required from your side. Firebase will automatically handle the changes to APNs certificates. For peace of mind, you can review Firebase’s documentation or contact Firebase Support if needed.