ImportExportTools Requires Restart
Adds some tools to import and export folders and messages
Allows bidirectional access to Google Calendar
SQLite Manager Requires Restart
Manage any SQLite database on your computer.
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
MinimizeToTray revived Requires Restart
Minimizes windows into the system tray.
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
QuickNote Requires Restart
A simple note-taking extension.
Mail Redirect Requires Restart
Allow to redirect (a.k.a. "remail") mail messages to other recipients.
UnMHT allows you to view MHT (MHTML) web archive format files, and save complete web pages, including text and graphics, into a single MHT file in Firefox/SeaMonkey.
If you are the kind of person who likes to have total control over every aspect of your downloads, "Download Status Bar" is the add-on you are looking for.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
LightningButton Requires Restart
Toolbar-Buttons for Lightning; advances the feature to get rid of the tabbar.
ProfileSwitcher Requires Restart
It adds some options to launch easily other profiles and the Profile Manager.
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
Dafizilla Table2Clipboard Requires Restart
Mozilla applications allow to select rows and columns from a table simply pressing Control key and picking rows/columns with left mouse button.
The selection can be copied to clipboard but the original table disposition is lost making ugly results wh...
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.
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
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.
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.