I now reverted to these steps: https://github.com/arnesson/cordova-plugin-firebase/issues/1081#issuecomment-503135862, only had to remove and add the android platform again after I added his version of the cordova-plugin-firebase. Then these versions were in android/build.gradle:
classpath 'com.google.gms:google-services:4.2.0' // google-services dependency from cordova-plugin-firebase
classpath 'io.fabric.tools:gradle:1.25.4' // fabric dependency from cordova-plugin-firebase
I can finally build again but I am worried about who will maintain the fixed cordova-plugin-firebase