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It is now much easier to switch between the mail and calendar parts of Thunderbird thanks to a user interface redesign
A redesigned event/task dialog has been enabled to gather feedback from the community. It offers a much cleaner user interface and additional functionality including:
Events/Tasks can be created in different timezones
Attendees can be added from a local or corporate LDAP directory
Free-Busy information is available for users of the Sun Java Calendar Server
Integrated display of recurrence patterns, reminders and attendees
Custom recurrences and reminders
Clear separation between tasks and events
The new event summary dialog lists all the details of tasks and events for calendar which are read-only
The Today Pane gives a quick overview over your current tasks and upcoming events. The Today Pane can be enabled/disabled by a toolbar button
Colliding events (events in the same time slot) are shrunk so that they fit into the display of a day. This no longer affects other events on the same day
Localized releases of Lightning and Sunbird in Chinese (simplified), Georgian, Lithuanian, Portuguese (European), Spanish (Argentina) and Turkish
Event invitations can be sent and received via iMIP/iTIP (Lightning only)
Working hours are back
Automatic migration of data in Sunbird 0.2, iCal.app, and Evolution
Much improved printing functionality
Better integration of Lightning into Mozilla Thunderbird (Printing, Undo/redo, Copy/paste)
Support for Google Calendar (via the Google Calendar Provider extension)
and much more. A full list of changes that have gone into this release can be found on The Rumbling Edge weblog.