unreadCount property represents the number of unread messages belonging to the thread.
var unread = instanceOfMozMobileMessageThread.unreadCount;
This is a non-standard implementation, but it is discussed at W3C as part of the System Application Working Group.
|Messaging API||Editor's Draft||Editor Draft (WIP).|
For obvious reasons, support is primarily expected on mobile browsers.
Supported in Firefox OS 1.0.1.