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.
There is GameSparks with a plugin. It is similar to Firebase but it is more focused on games unlike Firebase. There is a plugin, although it is kind of limited. So, you'd still have to code in JavaScript, to extend the limited capabilities of the plugin.
I currently use PlayFab, although, this is purely using it through Javascript. There is no plugin for it.
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.
Out of all, I would recommend to use PlayFab but you are going to have to fully use JavaScript to implement this.
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.
So, you still have to use JavaScript to extend the features but minimally.
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.