No solutions posted here worked for me. A wonderful person opened a pull request in the cordova-firebase-plugin official repo and it works.
Steps I did:
1 – Remove cordova-firebase-plugin with ionic cordova plugin remove cordova-plugin-firebase
2 – Install: ionic cordova plugin add cordova-plugin-firebasex
3 – rm -rf node_modules/ plugins/ platforms/android package-lock.json
4 – ionic cordova platform add android && npm install
And now it’s working.