Download Thunderbird
Currently, there is only an experimental Thunderbird build for Windows, Linux and MacOSX available. In the future, there will be both nightly binaries and milestone releases. Download the experimental build here:
ftp://ftp.mozilla.org/pub/thunderbird/nightly
Please note that Thunderbird is currently not even alpha quality software. Use this test build at your own risk.
How To Install
For windows, unzip the file into a directory of your choosing, then launch thunderbird.exe. If something goes wrong or if Thunderbird doesn't start, you can try to create a new profile by using the Profile Manager.