Mozilla Messaging has release the first Release Candidate for the upcoming Thunderbird 3.1. This release is intended for extension developers and beta testers. New features in Thunderbird 3.1 that require feedback include:

• New Quick Filter toolbar
• New Migration Assistant
• Saved Files Manager
• Several fixes to improve upgrading from Thunderbird 2
• Several design improvements and corrections to the interface
• Stability, memory, and password handling improvements

More information and download available via Mozilla Developers News.