These addons solve specific issues with the vanilla Thunderbird installation. They are all highly recommended to make your life with Thunderbird spiffy.
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).
ReminderFox displays and manages lists of reminders and ToDo's. ReminderFox makes sure you remember all of your important dates via easy-to-use lists, alerts, and alarms, right in your browser without the need for a separate calendar program.
This extension is for those TB3 users who want the multiple-columns view that TB2 allowed
British English Dictionary R1.19 for Firefox, Thunderbird and SeaMonkey by David Bartlett.
To enable, right-click in a multi-line text box, select "Languages", then "English / United Kingdom".
Search and remove messages which are the same as other messages in your mail folders (referred to as duplicates or dupes).
If you have duplicate messages (e.g. from merging folders or downloading messages several times) you can use this tool to find and remove them from the selected folder and its subfolder by rightclicking on a folder and choosing "Remove duplicates..."
The "Swiss army knife" of folders. Turn your favorites into tabs - open folders and subfolders, move mails without scrolling around or searching. powerful + easy.
See new mail at a glance after automatic filtering - supports quickFilters addon!
Select text and it's automatically copied to the clipboard. Also pastes on middle click.
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
This Extension implements the ManageSieve protocol for securely managing Sieve Script on a remote IMAP server...
This Add-on allows to have HTML temporarily allowed in the currently displayed message by only one click. When switching to another message, it'll be shown automatically again in plain text or simple html mode (if this is your default mode).
Toggle word wrapping and PRE element wrapping in the message composition and HTML display windows, respectively.
Unclutter the "Tools" menu by moving new items to a "More Tools" menu.