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.