Implementing javascript code in to xul/ thunderbird addons

I want to build thunderbird add ons for encrypt/decrypt email. I have the javascript code but i dont know how to use xul to make it work. Here is the javascript code in html :

http://jsfiddle.net/mindoftea/bud9V/

I need a way to use that javascript code into mozilla thunderbird addons.

I'm sorry if my English is not good. Thank you.