You can use Firebase (and they have a free tier to get started) but you won't have the benefit of integration with itch.io's login system. Users will have to create a new account, specifically for your game.
I already use PlayFab to save data. But I don't want users to create a separate account. Very convenient to use one user name in itch.io and game. And login user in PlayFab automatically. That's why integration with itch.io's login system will useful for me.