The Thunderbird 3.0 Release Candidate is now available for download. This milestone is focused on providing a preview of the functionality provided by the new features and changes that will be included in Thunderbird 3.0. New features in Thunderbird 3 that require feedback include:

• New Search Tools
• Tabbed Email
• Message Archiving
• New Mail Account Setup Wizard
• Improvements for Developers

Source: Mozilla Developers News