Thunderbird 3 Release Candidate is a preview release of the next major version of Thunderbird and it is being made for testing purposes only.
Thunderbird 3 is based on the Gecko 18.104.22.168 platform including some major re-architecting to provide improved performance, stability, web compatibility, and code simplification and sustainability. There are over 100 changes in this release, many laying the groundwork for future changes.
Internet and communication
Windows, Linux, Mac
Product page: here