Along with Mozilla Firefox 15, Mozilla also released Thunderbird 15 which incorporates several new features, such as the new instant messaging. It allows uses to talk to several IM protocols, such as GTalk, IRC, and XMPP. The Do-Not-Track features has been included as well.
A new menu and toolbar design has been included so that future releases will have the same and consistent theme along with Firefox releases.
More information about this release can be seen on the Release Notes