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.