Mozilla's Firefox 8.0 hit the web yesterday and while the web browser side of Mozilla is enjoying their time in the lime light, Thunderbird and Lightning have received updated versions, too. Mozilla added Thunderbird to their rapid release schedule a little while back, which means users will receive frequent, periodic updates.
Thunderbird 8.0 highlights include improved speed and responsiveness, add-ons installed by third party programs are now disabled by default, new search and find shortcuts are included, improved accessibility of the attachment list, folder switching pane widget has been removed, numerous platform fixes to stability and a number of security issues have been fixed. It's also based on the new Mozilla Gecko 8 engine.
Additionally, Lighting, which has been in beta for a few years now, has finally been polished and given the green-light to have version 1.0 attached to its name. Keep in mind, Lightning 1.0 is only compatible with Thunderbird 8.0, which means you'll need to upgrade.