The cross-platform front end. The part of Mozilla that implements the user interface of mozilla. Everything that makes a browser a browser except the rendering of pages is considered xpfe (bookmarks, toolbars, menus etc).

(Cross Platform Front End) a suite of technologies used to create applications that will work and look the same on different computer operating systems. A widely used XPFE application is the Mozilla web browser and its derivatives, such as the Netscape web browser in version 7 and later. The primary technologies used in creating XPFE applications are Javascript, Cascading Style Sheets, and XUL.

See: the XPToolkit.