New features include:
New Search with Advanced Filtering Tools

• New Search with Advanced Filtering Tools

Search results now include advanced filtering tools. You have the option to filter your results by sender, tag, attachments, people, folder, and mailing list. You can also filter your email using the timeline tool.

• New Global Search Field with Autocomplete

When typing in the Global Search field, Thunderbird autocompletes against your address book. You have the option of searching everywhere or filtering against different parts of the email such as by subject or by sender.

User Experience Improvements

• New Mail Account Setup Wizard

The new Mail Account Setup Wizard matches against a database of email settings from popular mail providers so that you will only need to provide your name, email, and password to set up new mail accounts.

• Redesigned Mail Toolbar

The Mail Toolbar is redesigned to include the new Global Search bar. Buttons such as reply, forward, delete, junk are part of each email message. You can add those buttons back to the main toolbar by customizing the toolbar.

• Tabbed Email Messages

Double-clicking or hitting enter on a mail message will now open that message in a New Tab window. Middle-clicking on messages or folders will open them in a Tab in the background. When quitting Thunderbird, visible tabs will be saved and will be restored when you open Thunderbird the next time. There is also a new Tab menu on the Tab toolbar to help you switch between Tabs.

• Smart Folders

The folder pane offers a Smart Folders mode which combines special mailboxes, like Inbox, from multiple accounts. Smart Folders is now on by default.

• New Message Summary View

Selecting multiple messages will give you a summary view of the emails you have selected.

• Column Headings

The column headings that are displayed and the order in which they are displayed can now be set on a per-folder basis.

• Message Archive

You can now file messages from your Inbox or other folders into the new Archive folder system.

• Activity Manager

The Activity Manager records all the interactions between Thunderbird and your email provider in one place.

• 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. You can also add a photo for contacts in your address book.

• Improved Gmail Integration

Better recognition and integration of Gmail's special folders such as Sent and Trash including non-English versions of Gmail. Thunderbird also uses All Mail as the Archives folder.

• For Windows Vista users, Thunderbird 3 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 is now integrated with Spotlight, can import from Mail.app, read your OS X address book, and use Growl for new mail alerts.

Performance Improvements

• IMAP Folder Synchronization

Thunderbird will download IMAP messages by default 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 / 'Synchronize & Storage'.