Thunderbird 3, which is currently available as a beta version, promises to be an excellent desktop email program far superior to the current version. It comes with many exciting features like powerful search options, archiving, tabbed email reading or message summaries. Most of the add-ons that are currently available at the Mozilla Thunderbird website are not compatible yet with Thunderbird 3. The following seven add-ons on the other hand can already be used with Mozilla Thunderbird 3.
The situation will likely change with the final release of the email client. It is on the other hand likely that most Thunderbird add-ons will function correctly if their compatibility is edited manually or by using an add-on like the Nightly Tester Tools.
Buttons [link]

The Buttons add-on adds several new buttons to Thunderbird 3 including buttons to display the next or previous email (as opposed to displaying the next or previous unread email which is the default option in Thunderbird), SMTP server selection, delete junk button that marks the message as junk and deletes it afterwards, HTML and Image on and off buttons or a delete thread button that deletes the entire thread of messages.
Hide Menubar [link]

The Thunderbird 3 menu does not usually need to be displayed all of the time. Especially users with lower screen resolutions might be interested to save screen estate to work more comfortable in the email client. Hide Menubar is a straightforward Thunderbird 3 add-on that hides the Thunderbird 3 menubar automatically. The alt key can be used to switch between hiding and displaying the menubar.
gContactSync [link]

This add-on makes it possible to synchronize contacts between the Thunderbird email client and one or multiple Gmail and Google accounts.

Smooth Wheel offers the ability to fine tune the mouse scrolling smoothness in Thunderbird. The default smoothness does not satisfy many Thunderbird users (and actually Firefox users as well) and this is the add-on to customize the smoothness for a better experience.
Remove Duplicate Messages [link]

Duplicate emails are usually experienced during connection errors or if the mail program or mailboxes got damaged. This add-on cleans duplicate emails easily offering to move or delete them in the email program.
FiltaQuilla [link]

FiltaQuilla adds new filters to Thunderbird extending the automatic email processing options in the email client. It is for example possible to launch or run a file, append to subject, train the email program as good or junk and mark the email as unread or replied.
Foxclocks [link]
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Can display multiple clocks in the Thunderbird interface.
Know of any other Thunderbird 3 add-ons that should be in this list? Let us know in the comments.

Thunderbird Delete Confirmations
Thunderbird Filter Additions
Fix Slow Or Hanging Thunderbird Email Client
Switch To Compact Headers In Thunderbird 3
Email Client: Thunderbird 2.0.0.21
Fix Thunderbird Junk Filter Not Working Anymore