I actually said that Cordova Firebase might really work since Cocoon.IO multiplayer works on Canvas+ because it is made as a Cordova Plugin. Although, I didn't test it yet, so I can't be certain.
To be honest though, I don't really recommend Firebase that much. Unless you want to rush into making and finishing your game and avoid programming. And use the most feature-packed and available, back-end service. Then it is more than fine.
I don't think PlayFab is practical to make a plugin of. It is very volatile and might change anytime and the reference list is long just like Firebase.
There is GameSparks, nearly the same as PlayFab. And also like PlayFab, it is designed for games. We do have a plugin for this on Construct 2 but it is limited.
But if you have little experience in programming or in a rush to develop your game. Then, Firebase is fine. Since, it have so many libraries/plugins online even a few are available on Construct.