subject property is a string representing the subject of the message (in many cases, its title).
var subject = instanceOfMozMmsMessage.subject;
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.