My favorite and some localized add-ons for Thunderbird, Sunbird/Lightning.
Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
Allows bidirectional access to Google Calendar
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
Access to Google contacts and synchronize them with Thunderbird address books.
LookOut is a plugin which allows Thunderbird to interface with Microsoft's mail tools by decoding metadata and attachments encapsulated/embedded in a TNEF encoded attachment (aka winmail.dat).
Japanese to English/German/French/Russian dictionary. Just hover the mouse on top of a word, and a popup appears. Automatically de-inflects verbs and adjectives. Detailed kanji view...
Collector's Note ポップアップ表示してくれる辞書。
Displays a menu item for switching the application locale.
Search and remove messages which are the same as other messages in your mail folders (referred to as duplicates or dupes).
The lightning-exchange-provider addon allows you to use Microsoft Exchange calendars in Thunderbird, via Exchange’s web services.
This addon only works with Microsoft Exchange 2007 and above. Exchange 2003 is not supported.
Allow to redirect (a.k.a. "bounce") mail messages to other recipients
Collector's Note 日本語改造版。http://www.geocities.jp/chimantaea_mirabilis/mailredirect/index.html
Extension to mimic Outlook Express Stationery.
Allow you to use HTML files as templates for mail. Also allow custom template in replies or forwarded mail!
Collector's Note メッセージのひな形管理。
Add symbol in header plane to switch between compact and expanded view. Options to switch between one or two lines compact view, which buttons should be displayed in header pane. Linkify RSS subject. Optionally darkens header if message has focus.
Collector's Note Thunderbird 3 用のメッセージヘッダの表示をカスタマイズするアドオン。
Displays the address books in a sidebar in Thunderbirds 3-pane-window and can be toggled with the F4 key or a toolbar button.
Visit my Contacts Sidebar site for recent versions and news.