Thunderbird 9.0 ßeta has the following changes:
- An express consent system has been added for users to submit (if they want) performance data to Mozilla.org, to improve future versions of Thunderbird.
- Handling of attachments is improved.
- Windows users, you can hide the Menu Bar (and show it again using the "Alt" key).
- Various fixes and improvements to the user interface.
Data taken from Mozilla.