These are just the Thunderbird addons that I use on a day-to-day basis
Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
Synchronizes contacts & groups between Thunderbird or SeaMonkey and Google Contacts. Imports Facebook friends, contacts you follow on Twitter, and more.
British English Dictionary R1.19 for Firefox, Thunderbird and SeaMonkey by David Bartlett.
To enable, right-click in a multi-line text box, select "Languages", then "English / United Kingdom".
Displays icon for user agent of received mails.
Display birthdays from the Thunderbird addressbooks as events in Lightning.
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.