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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.