Some cool new features in3.0, 3. It seems fairly stable, too.
- Auto archiving
- Messages and threads can be opened persistent tabs (like Eudora)
- Global indexing and search
- Thread summary when looking selecting an unexpanded thread
- Attachment reminder — YES! If you use the word "attached" or "attachment" in your message and there’s no attachment, Thunderbird will warn you when you try to send the message.
- Virtual folder merging to create a virtual inbox across multiple accounts.
The full list is at http://kb.mozillazine.org/Thunderbird_3.0_-_New_Features_and_Changes.