Spicebird isn't as relevant as it was when the project started. A few months after we started working on Spicebird, Mozilla Messaging emerged and development was active again. Many features that kept Spicebird stand out started getting introduced into Thunderbird. Thunderbird began evolving so rapidly that it was really hard for Spicebird to keep up.
The latest source code is available for download, but it is based on a pretty old version of Thunderbird. It will be great if you can drop us a line about how you plan to use the source code — some of us might even help you in the new project!
There are plans to push some of the Spicebird features as Thunderbird addons