charsAvailableInLastSegment property represents the total number of characters available for the last segment.
var last = instanceOfMozSmsSegmentInfo.charsAvailableInLastSegment;
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.