Better Gmail 2

If you want to add a few features and skins you could download a firefox extension called Better Gmail. Now here's an updated version of the extension which adds even more functions and features to your Gmail. Better Gmail 2 is essentially a compilation of Greasemonkey user scripts that add features to Gmail. So your only requirements apart from Firefox would be Greasemonkey extension installed.

Download Better Gmail 2.

Further details available at LifeHacker.