FUEL
From MDC
This article covers features introduced in Firefox 3
FUEL is a JavaScript Library designed to help developers build extensions using terminology and interfaces that are familiar to them. FUEL is new in Firefox 3 and will be backported to Firefox 2 as well.
FUEL is about making it easier for extension developers to be productive, by minimizing some of the XPCOM formality and adding some "modern" JavaScript ideas. We want to start with areas that will provide the most benefit.