Thunderbird:Extensions
Mozilla Add-ons
Thunderbird Add-ons at addons.mozilla.org is great way to start browsing Thunderbird extensions.
Writing An Extension
See the Mozilla Developer Center for help with writing extensions. There is a list of links to helpful development tools. The Thunderbird page has a tutorial, some examples, and links to other sites with resources for Thunderbird extension developers.
STEEL
STEEL, by jminta, stands for a proposed Scriptable Thunderbird Easy Extension Library, a FUEL-like set of interfaces to facilitate Thunderbird extension development.