Since the recent Facebook re-design, there are not many themes or skins out there that work with it. However, I have found one that is fully compatible and (for some) a much more pleasant viewing experience, the Vista Ultimate Skin for Facebook.
Vista Ultimate Skin for Facebook – Want to make Facebook look a ‘bit more Vista-like? This user style will give Facebook the look and feel of Windows Vista.
Also, if you want just the Vista-like taskbar inside of Facebook, you can pick up this user style.
For these two user styles to work, you have to have the Stylish extension for Firefox installed. Once you do, be sure to check out our entire archive of Stylish tweaks and tips. Also, if there are any Firefox Facts fans on Facebook, be sure to look me up and add me as a friend. Have any favorite user styles you like to use with Facebook to tweak the interface?