Add-ons

ImportExportTools by Paolo "Kaosmos"

Adds some tools to import and export folders and messages

Provider for Google Calendar by Philipp Kewisch

Allows bidirectional access to Google Calendar

German Dictionary by KaiRo

German Dictionary (new Orthography) for spellchecking in Mozilla products

Spanish (Spain) Dictionary by tnarik, RickieES

Spanish/Spain Dictionary

Quicktext by John Bieling

Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.

LookOut by Aron Rubin

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).

Thunderbird Conversations by Jonathan Protzenko, Standard8

This addon provides a conversation view for Thunderbird, grouping messages together and allowing you to reply "inline" thus providing a more efficient workflow.

Collector's Note I think this is the most important Add-On for Thunderbird. It organizes your email threads the same manner has does GMail. Capable of finding related mails even in different folders it is moreover a great replacement for ThreadVis add-on.

Signature Switch by Achim Seufert

Switch the signature on/off or choose a new one from your predefined set.

Stationery by Arivald

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!

Addressbooks Synchronizer by Günter Gersdorf

Synchronizes selected addressbooks with other instances of thunderbird
using local files, webdav servers or imap servers

Calendar Tweaks by DW-dev

Allows several tweaks to the Thunderbird/Lightning calendar user interface including new selection styles and repeating event icons. Mimics the colors and styles found in MS Outlook.

Color Folders by fisheater

Color your Folder Treeview Icons!

Thunderbird Message Filter Import/Export Enhanced by kounavi

Thunderbird Message Filter Import/Export Enhanced is a compatibility port of Wind Li's add on.
Original add on here: https://addons.mozilla.org/en-US/thunderbird/addon/thunderbird-message-filter-imp/

Thunderbird Message Filter Import/Export by Wind Li

Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.

eCleaner by flasresizerdeveloper

Clears preferences left by extensions after uninstallation