Here is a list of helpful MozillaThunderbird Extensions (aka Plugins) that I found useful myself.
Enigmail
OpenPGP message encryption and authentication. Requires Platform-Specific Enigmime extension, http://enigmail.mozdev.org
Download URL: http://update.mozilla.org/extensions/install.php?id=71&vid=743 1?
Forumzilla
Forumzilla is an extension to Thunderbird and Mozilla Mail for reading news and blog feeds (all flavors of RSS plus Atom). It lets you read feeds using the familiar three-pane mail interface.
Download URL: http://update.mozilla.org/extensions/install.php?id=150&vid=245 1?
Foxytunes
Forumzilla is an extension to Thunderbird and Mozilla Mail for reading news and blog feeds (all flavors of RSS plus Atom). It lets you read feeds using the familiar three-pane mail interface.
Download URL: http://update.mozilla.org/extensions/install.php?id=133&vid=484 1?
No New Window on Double Click
Prevents Thunderbird from opening a new window when you double click a folder.
Download URL: http://update.mozilla.org/extensions/install.php?id=150&vid=245 1?
1? Right click and save as, because if you use MozillaFirefox it assumes it is an extension for itself.
No other page links to MozillaThunderbirdExtensions yet.