In order to send a push notification in iOS from Firebase, you need also to upload the push .p12 (APS called in developer.apple.com)
Before, you need to have generated it following these instructions. (it's the same used in mobincube notifications).
In this way, already in firebase in the Project Overview area, go to the Cloud Messaging part and in the iOS app Configuration , and proceed to configure the app.
Once there in the part of APNs Certificates you must upload the Apple APNs Certificate.
Then, once the iOS app has been selected, you need to upload the production .p12 file that you exported from your mac previously.
Important: Remember to leave the password of the Certificate blank, that is, without a character.