The next major release of Thunderbird, version 38, is now in beta and available for testing. You may download Thunderbird 38.0b1 here.

This version of Thunderbird is the first that is mostly managed by volunteer community members rather than by Mozilla staff. We have many new features, including:

Message filtering when a message is sent or archived

File-per-message local storage available for new accounts (maildir)

Contact search over multiple address books

Internationalized domain names for RSS feeds

Allow expanded columns to the folder pane for folder size and counts

Release notes are available here.

There are still a couple of features missing from this beta that we hope to ship in the final version of Thunderbird 38. Those are: