Thunderbird 8 has been released on November 8th, 2011. Current Thunderbird 7.0.1 users can update via Help > About Thunderbird. Lightning users, after you have updated to Thunderbird 8.0, your calendars will be broken. To fix this go Tools > Add-ons and then in your add-ons list, find and right-click on Lightning, selecting Find Updates. Lightning will update to version 1.0 and once you restart Thunderbird, your calendars will be working again.
Users can also manually download Thunderbird 8.0 via getthunderbird.com and then install over your existing Thunderbird 7.0.x install. For more information as to what’s new in Thunderbird 8.0, see this post. The next scheduled released is Thunderbird 9.0 on December 20th, 2011.