Single Domain Requires Restart
Warns if you send mail from or to a specified domain. If there are multiple accounts in your profile, you can isolate one so that you can't send mail to that domain from outside or vis versa without warning.
FileRun for Filelink Requires Restart
Add FileRun support to Thunderbird's Filelink feature.
HtmlHighlightr Requires Restart
Provides source code highlighting in editor's "Insert HTML" dialog.
SenderFace Requires Restart
SenderFace enables you to associate images (faces) with Email senders (from...
Free/Busy Requires Restart
Publish free/busy calendar file (*.ifb) via ftp or to a file. View other people's free/busy info in meeting organizer window. Integrate Thunderbird and Outlook calendars without Exchange server.
Attachment Manager Requires Restart
An extension to manage attachments across multiple emails.
Opacus SugarCRM (5.5 to 6.1) Professional Requires Restart
Enables archiving of emails into SugarCRM (v5.5 to v6.1) from Thunderbird. Important: This add-on requires a licence key (request a trial from within the extension). Please visit http://www.opacus.co.uk/opacus-thunderbird-extensions for details.
Resize Header Requires Restart
An addon to let you resize the message header pane.
Filter Button Thunderbird Requires Restart
Adds a button to run...
ltnPlus Requires Restart
Support printing of tasks for Lightning 0.8/0.9.
Up to version 0.2:
Enhancements to Lightning that are known from Sunbird: Unifinder (for events and todos) and moving MiniMonth to Tabpanel
Tidybird Requires Restart
Organize email into folders quickly and easily.
GNOME keyring password integration Requires Restart
Extension that allows Firefox and Thunderbird to store passwords in Keyring, Gnome's default password store.
TheRealURL Requires Restart
Get URL information: expand short URLs, get title and content type information without visiting the site.
UPDATE, AUG 2013: THIS EXTENSION IS NO LONGER SUPPORTED. The source is open - if you wish to take on the development, you're welcome.
SmartFilters Requires Restart
Tired of writing a lot of filters? Install SmartFilters now to organise your mailbox automatically. It can sort out your mailing lists and robot emails.
Reject Button 1.0 Requires Restart
Annoyed by receiving mails with missing or wrong subjects or meaningless message bodies? Simply reject them with the Reject Button Add-On and enforce proper e-mail etiquette at the cost of just a few friendships.
Event-to-task conversion activator Requires Restart
Activates the Event-Task conversion ability...
MozLab Requires Restart
Remote shell and test-driven development tool for Mozilla developers
Tb AutoSave Extension Requires Restart
You can now autosave messages from TB upon...
EmailPicky 4 Requires Restart
Grab email addresses from emails from any selected Email Folder and add them to your contacts. (Email Crawler / Email ID Grabber / Bounced email collector) - Thunderbird Plugin
Note: It doesn't create duplicates, but shows as 'xxx messages added'.