Firefox only (Windows): Firefox theme Daum Blue is not just clean and simple—it also looks great and blends in well with your Windows 7 or Vista desktop.
The theme works well across the board, and even includes smaller versions of the buttons if you prefer a smaller toolbar. Daum Blue is a free download for Firefox, but the theme is only compatible with Windows for the moment, so Mac and Linux users are left out in the cold.
If the bright colours are too much for you to handle, learn how to consolidate Firefox's chrome to save some space, or check out the Aero Geek Firefox customisation that uses Windows Vista Aero effects.
Daum Blue [Mozilla Add-ons]