As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements handled by Mozilla.
Thunderbird is a free, open-source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a Web browser.
It turned out to be very comfortable to use. One more pleasant thing about the application is the fact that it's free. The tab feature really makes dealing with letters comfortable.
Totally awesome email application, and it's completely free on top of everything!!
It would appear the major version number jump is due to the fact that Thunderbird now based on the Mozilla Gecko 6 engine