XPFE & XPToolkit permit consistent '''creation''' of user interfaces across the 3 major platforms. As for why the two names: "'XPToolkit' is rather synonymous with XPFE. Though the former term seems more concrete than the other, and therefore is not an exact replacement, no one is completely certain why we have both." [http://developer.mozilla.org/en/docs/Introduction_to_XUL#Terms]. From here on in we'll refer to XPFE (because it's shorter to type). Note: This is also commonly refered to as Toolkit.