Up & Coming Extensions
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.
Open about:config to change hidden settings that cannot be edited from the Options window. All Mozilla based programs have settings that can not be changed in the options window.
Color Text Compose Buttons Requires Restart
Adds Text Color Buttons in Compose Toolbar and More: Pixel FontSize, Strikethough, Sub & Superscript, Lower & UpperCase. Also provide Tools: Insert iframe, Launch MathType, Maths & Greek Symbols, Load Template Variables and Duplicate Compose Window.
Unsubscribe from Mailing List Requires Restart
Allows recipients to cancel their newsletter subscriptions in the thunderbird interface.
Expression Search / GMailUI Requires Restart
Powerful message searching through expressions. Type "from:fred to:tom" to see all messages from Fred to Tom in the current view. Support 'regular expressions' and 'click to search'.
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
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.
AttachExtraTools Requires Restart
This extension gives more options and tools to attach files from compose window.
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.
Remove Duplicate Messages Requires Restart
If you have duplicate messages (e.g. from merging folders or downloading messages several times) you can use this tool to find and remove them from the selected folder and its subfolder by rightclicking on a folder and choosing "Remove duplicates..."
NewScrollbars (aka NoiaScrollbars) Requires Restart
The "NewScrollbars"-extension replaces default scrollbars of Firefox and Thunderbird with colorized ones (partly) known from "Noia themes".
Account Colors Requires Restart
Associates user-defined colors with accounts and identities. Applies these colors to accounts, identities, folders, message list, message headers and compose headers. Users will always know which accounts/identities they are working with.
Toggle Headers Requires Restart
Allows to switch between the normal header view and viewing all headers when viewing a message by using a keyboard shortcut (H).
Exit Button Thunderbird Requires Restart
Adds a toolbar button to exit Thunderbird. Go to View menu, Toolbars, Customize... and drag the icon where you want...
G-Hub Lite Requires Restart
Add Google Tabs to Thunderbird! Services supported: Voice, Calendar, Drive, Docs, Sheets, Slides, Hangouts, Plus, Photos, Contacts, News, Maps, & many more. Need Google Apps? Check out G-Hub Pro: http://www.schiffner.com/index.php/software/g-hub-pro
DL for Thunderbird Requires Restart
Allows to convert large attachments to links automatically without 3rd party providers.
ThunderCal Requires Restart
ThunderCal simply gives you a toolbar button that you can use to run Google Calendar inside Thunderbird. Inspired by the ThunderWave add-on.
Contact Tabs Requires Restart
Contact Tabs enhances Thunderbird to give you quick access to all your contacts directly from Thunderbird's main window: Use the powerful search field to find your contacts and open them in a new tab for viewing and editing.
Super Date Format Requires Restart
Fully customize the date format of the Date/Received column.