The following items describe problems and questions that users have encountered when upgrading from Thunderbird version 2 to version 3 (and 3.1). (Click the links to access the relevant help articles where applicable.)
Master password requested but not set: After upgrading to Thunderbird 3.0, some users have found that Thunderbird requests a Master Password even when no Master Password was set in Thunderbird 2
Add-on version compatibility: Add-on authors are responsible for updating their add-ons to ensure that they work with new versions of Thunderbird. Please visit the Mozilla Add-ons site to check add-on version status
Unable to authenticate to SMTP server: After upgrading to Thunderbird 3, some Thunderbird users are getting an error message that says "An error occurred sending mail: Unable to authenticate to SMTP server ..." e.g. for BellSouth email users "Sending of message failed. An error occurred sending mail: Unable to authenticate to SMTP server mail.bellsouth.net. It does not support authentication (SMTP-AUTH) but you have chosen to use authentication. Uncheck 'Use name and password' for that server or contact your service provider."
Using the new Global Search functionality, and understanding that Thunderbird might initially be slow while it indexes messages
When new POP accounts are created, by default messages on the server are deleted after 14 days. To change this setting, click on Tools | Account Settings, then select the Server Settings page beneath the account name in the left panel. Alter the values in the Server Settings section of the page according to your preferences.
Modify the toolbar: Some people prefer the earlier version of the Thunderbird toolbar to the default version included in Thunderbird 3. There are three was you modify the toolbar to suit your preferences:
Revert to the old-style toolbar using the Migration Assistant (Help | Migration Assistant).
Manually customize the buttons on the toolbar (View | Toolbars | Customize).