Mozilla Lightning 0.1

The Calendar team is proud to announce the first official release of the new Lightning extension: Lightning 0.1 for Windows, Linux and Mac OS X. This is a major milestone on the road to an integrated calendar for users of the award-winning mail-client Mozilla Thunderbird 1.5. Thanks go to all developers, testers and other supporters of the project. More information and download instructions are available on the Lightning homepage."
Download:
Windows
Linux
Mac

Do not install with Firefox, right click and save it onto your desktop, then open up Thunderbird and go to Extensions and hit "Install", then find the file you downloaded and select it.