Thunderbird 3 Beta 1 is based on the Gecko 1.9.1 platform which has more than 10,000 updates including some major re-architecting to provide improved performance, stability, web compatibility, and code simplification and sustainability.

There are over 750 changes in this release, many laying the groundwork for future changes. Some notable changes include:
User Experience Improvements

* Tabbed Email

Email messages, folders, and calendars (with the Lightning extension installed) can be opened in tabs.
* Improved Message Reader View

This is the first stage of a series of refinements to the message reading experience. The first stage brings email controls closer to the area in which the user is operating.
* New Add-ons Manager

The new Add-ons Manager (Tools > Add-ons) can now be used to find, download, and install Thunderbird Add-ons which includes Extensions, Themes, and Plugins. Note that few Add-ons are compatible with this beta at the time of release, as Add-on developers need to upgrade them.
* Improved Address Book

If someone is in your address book, it is indicated by a new star icon which you can click to edit contact details inline. If they are not in your address book, you can add them with one click of the icon. A new birthday field allows you to keep track of your friends' birthdays.
* For Windows Vista users, Thunderbird 3 Beta 1 is now integrated with Vista search results. On first start, Thunderbird will prompt to install its indexing system into Windows Vista and you can choose to see Thunderbird email and news messages in Windows search results.

* For Mac users, Thunderbird 3 Beta 1 can import from Mail.app, read your OS X address book, and use Growl for new mail alerts.

Performance improvements

* Faster Message Loading for IMAP

Thunderbird will now download IMAP messages in the background to allow for faster message loading, and better offline operation. This feature can be enabled on an individual folder basis (via folder properties), or for all folders in an account, via Account Settings / Sync & Disk Space.