The Migration Assistant is a tool included with Thunderbird 3 that helps people who are upgrading from earlier versions to configure the latest version according to their needs and preferences. While it does not provide access to all the features and configuration options included in Thunderbird 3 (which can be accessed in the usual manner), the Migration Assistant includes the items that our beta testers most frequently wanted to change.
The Migration Assistant was substantially expanded in Thunderbird version 3.1. See the MozillaZine article "Thunderbird 3.1 - New Features and Changes" for information about the 3.1 version of the Migration Assistant.
The Migration Assistant is displayed during the upgrade process. You can re-run it at any time by selecting Help | Migration Assistant.
If your incoming email server uses the IMAP (rather than the POP) protocol, you can configure whether or not messages are synchronized to your local computer, or whether they are stored only on the server. This will allow you to access email when you are not connected to your email server, and also enables enhanced search capabilities in Thunderbird. If you synchronize your email, be prepared that it will take some time to download the messages and that the messages will use your local disk space.
With the Migration Assistant, you can enable or disable synchronization for all IMAP accounts, or enable it for only specific accounts.
Thunderbird 3 has a new toolbar that leaves more room for the new search interface. You can revert to the style used in Thunderbird 2 by pressing use original toolbar, or change back to the Thunderbird 3 style by pressing use new toolbar.
Unified folders (previously known as "smart folders") can be configured so that messages in multiple accounts are grouped together in a single view. For example, if you had two email accounts (one for work and one for personal use), you could use unified folders to show a single inbox with messages from both your work and personal accounts. (To view unified folders, make sure the "Unified Folders" panel is displayed on the left side of the main Thunderbird interface. If it isn't displayed, use the arrows to cycle between the views.)
Thunderbird 3 uses a new design for message headers, with buttons for frequently used actions.
If you prefer the compact header that was available in Thunderbird 2, you can download one of the several Thunderbird add-ons that provide alternate message header views.