Messaging is already here. Except it's called the Social Service API.
For example, there are two Facebook Services. You can go (right now) to the Mozilla Social Service Activations Portal, select Facebook Share, and click "Activate now":
https://activations.cdn.mozilla.net/en-US/facebook.html [Requires a Facebook account]
There used to also be a Facebook Messenger Service, but that appears to be gone for some reason:
https://support.mozilla.org/en-US/kb/how-does-facebook-messe...
There's developer documentation here:
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/So...