Add-ons

CuteButtons by ChoGGi

Adds icons to buttons and/or menus.

AutoPager by Wind Li

AutoPager automatically loads next pages when you reach the end of a page. It works on a ton of sites,Google,Yahoo..
It works well with most other add-ons adblock plus, WOT and most of the greasemonkey scripts.
You can also try AutoPagerLite.

SubSwitch by Tomasz Krajewski

Simplify email categorization - it will be easier for you and others to manage mail. Define the set of prefixes for all emails you send or let SubSwitch to autodiscover it. While writing insert prefix to the title - choose one from your set.

Mark All Read Button by Mark S Baines

Adds toolbar buttons to Mark All emails as Read, Mark Thread as Read, and Goto Next Unread Thread.

Extra Folder Columns by Joey Minta, Thunderbird Development Team

This extension is for those TB3 users who want the multiple-columns view that TB2 allowed

IMPORTANT: If you are using Thunderbird 38 or later, uninstall this addon. The functionality is in core now.

Forward by Onno Ekker

Add Inline / As Attachment option to Forward button and Message context menu.

Display Contact Photo by Samuel Mueller

Displays the contact photo/image/picture of the sender/recipient while reading or composing a message. Supports photos in a local folder and the 'Face' header for embedded photos. The add-on adds new generic photos and many styling possibilities.

MailHops by Andrew Van Tassel

MailHops maps the route an email took to get to you. Displaying the senders location, weather, user-agent and authentication used.

Priority Switcher by H.Ogi

Change priority of the received messages from the toolbar button and the context menus...

JunQuilla by R Kent James, Christopher Leidigh

1) adds column with percent likelihood that a message is spam, 2) extends junk status column to show why a message was classified as junk/good, 3) adds an Uncertain folder for messages needing user decision.

Sender Name by Takahiro Shinagawa

Folder Account by Christopher Eykamp, welpy-cw

Lets you associate user accounts and identities with specific folders. Great for multiple users who share TB, or for managing mailing lists.

Year view by fabrix

Year view for calendar.

More Snooze by Cyrille Nocus, peci1

This add allow to customize the snooze delay when snooze popup appears (lightning required).

Disable "You" by JasnaPaka

Addresses in the From/To/Cc headings are since Thunderbird 3 replaced with word "You" if they match any of your identities. This extension disables this function.