This is a small collection of my favorite addons, which I always install with any Firefox.
United States English Spellchecker for Firefox, Thunderbird and SeaMonkey by Kevin Atkinson and Geoff Kuenning.
To enable, right-click in a multi-line text box, select "Languages", then "English (United States)".
OpenPGP message encryption and authentication for Thunderbird and SeaMonkey.
Quicktext is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails.
Danish dictionary for Firefox, Thunderbird and other Mozilla products.
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".
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...