Mozilla stopped actively developing Thunderbird a few years ago. It's still being maintained by a small group of developers, but it hasn't received any significant new features in some time, and mostly just gets maintenance and bug fixes. That said, they have did commit to adding more developers to the project a few months ago, and do have some plans for new features in the next few months - https://blog.mozilla.org/thunderbird/2014/11/thunderbird-reo...