Provider for Google Calendar Requires Restart
Allows bidirectional access to Google Calendar
ImportExportTools Requires Restart
Adds some tools to import and export folders and messages
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 Requires Restart
Allow to redirect (a.k.a. "remail") mail messages to other recipients.
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.
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...
PrintingTools Requires Restart
Adds some options to print messages and addressbooks
Add-on has now been discontinued, see below.
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.
Xpunge Requires Restart
Empties the Trash and Junk folders, and compacts folders in multiple Thunderbird accounts.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
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.
MinimizeToTray revived Requires Restart
Minimizes windows into the system tray.
Add Signature Requires Restart
Simply add your signature manually (right mouse click) to your emails
IMAP Quota (Free Space) Requires Restart
This Thunderbird addon lets you customize the IMAP quota statusbar panel.
Auto Compress File Requires Restart
Compressor for attachment files. All platforms are supported (Windows, Linux and Macintosh).
ThunderKeepPlus Requires Restart
Google Keep for Thunderbird
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.
ProfileSwitcher Requires Restart
It adds some options to launch easily other profiles and the Profile Manager.