Lightning is the working project name for an extension to tightly integrate calendar functionality (scheduling, tasks, etc.) into Thunderbird.
Q. Is Lightning meant as a competitor to Outlook?
A. With Lightning, Mozilla Thunderbird will have a set of user features that is much more competitive with Outlook, especially in enterprise usage. The primary goal of Lightning is to provide a pleasant and productive user experience for both email and calendaring tasks, largely independent of specific competitors’ product plans. If the result of the Lightning project is an acceleration of users’ migration from Outlook to Thunderbird, though, very few tears will be shed.
Thunderbird with calendaring features would be very cool.