Extensions

Creating a chrome overlay

Learn how to add a GUI to extension.

Modifying the browser with an extension

Learn how to add to or override the functionality of the browser.

Understanding Preferences

Change preferences programmaticly.

Reference

Extensions at the Mozilla Developer Center has excellent instructions for setting up a development environment.

Jetpack

Mozilla Labs has developed Jetpack, a new way to modify the behaviour of Firefox. Learn how to write a Jetpack.