Thunderbird 3 Release Candidate 1 is now available

Thunderbird 3 Release Candidate is a preview release of the next major
version of Thunderbird and it is being made for testing purposes only.This release does not represent the final product. So do not use Thunderbird 3 Release Candidate in a production
environment.

New in Thunderbird 3 

Thunderbird 3 is based on the Gecko 1.9.1.5 platform
including some major re-architecting to provide improved performance,
stability, web compatibility, and code simplification and sustainability.

There are over 100 changes in this release, many laying the groundwork
for future changes. Some notable changes 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