In Thunderbird 11, the layout has been altered to move the tabs from below the main toolbar to above the main toolbar.
This is part of a larger project to refresh the Thunderbird user interface, and makes it possible to improve the context sensitivity of the toolbar.
New spam header providers
Various organizations offer services for analyzing messages for spam. When a message is analyzed by one of these organizations, the organization adds an additional "header" to the message. For each of your email accounts, you can specify an organization whose headers Thunderbird should "trust".
For example, if you use SpamAssassin to analyze your incoming messages, you can configure Thunderbird to designate messages as spam based on the headers added by SpamAssassin.
To configure the organization that Thunderbird will trust: