Windows 7: Critical bug fixed in Thunderbird

Mozilla updated its nearly-baked Thunderbird 3 Tuesday night, fixing one critical error and addressing three others. The critical bug fix affects the Windows, Mac, and Linux versions of Thunderbird 3 Release Candidate 2. It had caused the previous version to hang on shutdown, consume too much memory, and close all IMAP connections.

The other fixes in this version enable downloading new messages automatically by default for POP3 accounts, label the default theme as version 2.0, and fixes the "download more dictionaries" option, which had been failing to open properly.
Already a year off from its original release schedule,

Thunderbird 3 has also missed its most recent revised deadline, which had been planned for the end of November 2009. However, I've been using the beta and release candidate builds and have found them to be as stable as Thunderbird 2, but with far less memory hogging.

reghakr's NIOTE: I tried out version 3, it sucked.

Source:Critical bug fixed in Thunderbird | The Download Blog - Download.com