This test version includes numerous back-end improvements that users will probably notice only as performance enhancements, but there are two new major features worth noting.
There's a new activity manager that records all interactions between your e-mail provider and Thunderbird, making it far simpler to track down errors when you send or receive mail. There's also an entirely new system for archiving messages based on Gmail's "archive and forget it" method. The new beta offers the traditional multiple-folder-based solution, as well as the new dumping-ground style, which can be activated via the "A" hot key.
A key part of Gmail's success with its archive, though, has been its excellent searching ability. The newer, faster searching tool promised for Thunderbird is not yet ready, according to developers.
IMAP users now will see Thunderbird download their messages by default in the background. Mozilla hopes that this will improve offline operations and speed up message loading times, and the feature can be activated on a per-folder basis or for an entire account.
Mac users should see improved Growl notifications too.
One major setback to Thunderbird 3 was announced earlier in February: the Lightning plug-in, which adds calendaring options to the e-mail client, will not be baked into the next major release.
Citing numerous concerns, including continuing interface adjustments and internal debates about the importance of the calendar to e-mail, Mozilla has decided to let Thunderbird be more flexible for now. An updated Lightning extension will be available for Thunderbird 3.