Mozilla Lightning Project is a redesign of the Sunbird Calendar component. Its goal is to tightly integrate calendar functionality into Mozilla Thunderbird allowing it to easily perform email-related calendaring tasks.Sunbird is a standalone calendar application. It does not offer any kind of tight integration with an e-mail application, while Lightning offers calendaring features directly in the Thunderbird User Interface. Further integration features, such as e-mail invites or address book integration are planned for future releases.
To install Lightning you required Thunderbird 1.5 or later. More details