Writing an Extension for Firefox

Duffblog: Writing an Extension for Firefox

"One of the best things about Firefox and Thunderbird is that they have a well defined extension mechanism. If there's some feature you feel is completely missing, you can go ahead and add it. It's relatively easy to do — you don't have to fiddle about with a C compiler because extensions are mostly written in a combination of XML and ECMAScript."

Reference: digg.com