You can download the Thunderbird mail application and review its release notes at http://www.mozilla.org/
Here's an important note from upgrading from 0.6 to 0.7. Version 0.7 creates a new folder of settings and mail and does not use any of the current information. I got around this by copying everything from my user/Library/Thunderbird/Profiles/default/iqsvpfr5.slt folder to user/Library/Thunderbird/Profiles/default.3iz/, so I hope this information helps someone else.
Here's an important note from upgrading from 0.6 to 0.7. Version 0.7 creates a new folder of settings and mail and does not use any of the current information. I got around this by copying everything from my user/Library/Thunderbird/Profiles/default/iqsvpfr5.slt folder to user/Library/Thunderbird/Profiles/default.3iz/, so I hope this information helps someone else.