Today I set up a Mozilla build-environment on my Powerbook, and builded-ed Mozilla Lightning, which aims to add tightly integrated calendaring functionality to Mozilla Thunderbird, the mail/news client.
From hanging out in #calendar on irc.mozilla.org, it appeared that most Lightning dev was happening on Windows, which meant that there weren't any nightly builds for us Mac users.
Building was surprisingly easy... and unsurprisingly long. The attached shot shows what it looks like so far.
I'll flex it's functionality when I get time, and will post about that as well. If you're brave/stupid, and want a copy of the xpi, let me know.