With enabled Aero Glass effect in Windows Vista/7 makes Window buttons, borders look transparent. If you want the same Aero Glass effects that for Firefox browser windows, toolbars and menus you can use GlassMyFox Firefox extension.

Firefox with custom tabs with Aero Glass effect

You must enable Aero Theme for Windows 7 under Personalization and of course your Computer hardware and video card must hardware requirements for running Aero. After installing the addon , it enables Aero Glass effects for default Firefox skin. This extension makes menus, toolbars and Windows transparent.

GlassMyFox options