ImportExportTools Requires Restart
Adds some tools to import and export folders and messages
Provider for Google Calendar Requires Restart
Allows bidirectional access to Google Calendar
Quicktext Requires Restart
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
Mail Redirect Featured
Allow to redirect (a.k.a. "remail") mail messages to other recipients.
PrintingTools Requires Restart
Adds some options to print messages and addressbooks
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...
Add-on has now been discontinued, see below.
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.
MinimizeToTray revived Requires Restart
Minimizes windows into the system tray.
LightningButton Requires Restart
Toolbar-Buttons for Lightning; advances the feature to get rid of the tabbar.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
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.
Xpunge Requires Restart
Empties the Trash and Junk folders, and compacts folders in multiple Thunderbird accounts.
Blinking Alert Requires Restart
Adds blink feature to the new-mail alert.
You can also disable auto-hiding of the alert.
FiltaQuilla Requires Restart
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.
Adds toolbar buttons to the customize toolbar window in several programs including Firefox, Thunderbird and SeaMonkey. Some of the buttons make commonly preformed actions quicker, others add new functionality.
Google Calendar Tab Requires Restart
Adds the Google Calendar web interface as a new tab directly into Thunderbird. Creating and viewing events works just as it would in a browser like Firefox
ProfileSwitcher Requires Restart
It adds some options to launch easily other profiles and the Profile Manager.
Auto Compress File Requires Restart
Compressor for attachment files. All platforms are supported (Windows, Linux and Macintosh).
Add Signature Requires Restart
Simply add your signature manually (right mouse click) to your emails