Thunderbird 3.1 is based on the Gecko 1.9.2 platform to provide improved performance, stability, web compatibility, and code simplification and sustainability.
New features include:
Faster Search Results and Quick Filter Toolbar
Faster Search Results
Message indexing is faster and provides users with faster search results.
Quick Filter Toolbar
New Quick Filter Toolbar lets you filter against search terms, tags, starred messages, address book contacts, new emails, and attachments.
User Experience Improvements
New Migration Assistant
The new Migration Assistant gives Thunderbird 2 users a way to choose the new features in Thunderbird 3.1 or to keep their current features and settings.
Saved Files Manager
New Saved Files Manager displays all the files you downloaded from your email to your computer.
Mail Account Setup Wizard
Hundreds of ISP settings have been added to make setting up Thunderbird easier. The 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.
Improvements to Stability, Memory, and Password Handling