Pode ser obtido no sítio habitual. Das release notes, as que me interessam mais:

* Message Tags: Create your own tags for organizing email. Messages can be assigned any number of tags. Tags can be combined with saved searches and mail views to make it easier to organize email.
* Advanced Folder Views: Customize the folder pane to show favorite, unread or recent folders.
* Easy Access to Popular Web Mail Services: Gmail and .Mac users can access their accounts in Thunderbird by simply providing their user names and passwords.
* Saved Search Folder Performance: Search results for saved search folders are now cached, improving folder loading performance.
* Find As You Type: Finds and highlights message text as you type.