Xulrunner is a single "gecko runtime" that can be used to bootstrap multiple xul + xpcom applications that are as rich as firefox and thunderbird.

xul is mozilla's xml based user interface language that lets you build feature-rich cross platform applications. these applications are easily customized with alternative text, graphics and layout so that they can be readily branded or localized for various markets. web developers already familiar with dynamic html (dhtml) will learn xul quickly and can start building applications right away.

xpcom is a cross platform component model. it has multiple language bindings and idl descriptions so programmers can plug their custom functionality into the framework and connect it with other components.