Allows bidirectional access to Google Calendar
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
ImportExportTools Requires Restart
Adds some tools to import and export folders and messages
DOM Inspector Requires Restart
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
Adds many new mail filter actions - launch a file, suppress notification, remove star or tag, mark replied or unread, copy as "read", append text to subject.
Display Quota Requires Restart
This extension will display the current status of your IMAP quota in thunderbird's statusbar and will warn you when you reach a configurable limit...
Empties the Trash and Junk folders, and compacts folders in multiple Thunderbird accounts.
PrintingTools Requires Restart
Adds some options to print messages and addressbooks
Add-on has now been discontinued, see below.
Mail Redirect Requires Restart
Allow to redirect (a.k.a. "remail") mail messages to other recipients.
FileLink provider for WebDAV Featured
Uploads your large attachments to your WebDAV account instead of sending them by email.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
Display Mail User Agent T Featured
Displays the mail program with which the selected mail was written
*This addon is compatible with Thunderbird after Web Extesnsions support. Since development and operation check are done on Daily, it may not work on normal Thunderbird.
MinimizeToTray revived Requires Restart
Minimizes windows into the system tray.
Google Keep for Thunderbird
LightningButton Requires Restart
Toolbar-Buttons for Lightning; advances the feature to get rid of the tabbar.
Calendar Tweaks Requires Restart
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.
Auto Compress File Requires Restart
Compressor for attachment files. All platforms are supported (Windows, Linux and Macintosh).
Moves the toolbar back above the tabbar.
URL Link is a small Thunderbird extension that allows you to select a non-URL in a mail/news message or web-page, and open it in a browser window.