For Vista users with Aero capability, you might want every software to have transparent theme or skin. What is Vista Aero? If you can make your taskbar, Start Menu, and all your windows to be transparent, that means you are using Vista Aero. If not, check your configuration and system capability. Almost all Microsoft softwares can automatically integrate the transparent theme of Vista but for Firefox, you'll need to use an add-on called Glasser. If the Glasser add-on doesn't work for you, use the AeroFX add-on. The AeroFX add-on is still in experimental phase therefore you'll need to register to use it. These add-ons can surely make your Firefox theme to match your Vista Aero theme.
Important note: Those add-ons only works with Standard Firefox Theme.