Mozilla Firefox is one of the best browsers on the market. It is designed to display the text of the page as quickly as possible. Probably, you are using Firefox to read this post. Anyways, if you have not yet installed this browser, you can click on button "Get Firefox with Google toolbar" at the left side to setup freely. This post will show you how to install themes for Firefox.
Ubuntu provides Mozilla Firefox themes such as Human, Tango, Industrial to make it look better integrated with default Ubuntu. To install these themes you run this command in your terminal.

sudo aptitude install firefox-themes-ubuntu

After the quick installation you have to restart Firefox to take effect. Go to Tools -> Addons -> Themes and take any available theme you like. In the snapshot, I have chosen the Tango theme.

If you are a Windows user and you want to install new themes for your Firefox, open Firefox, go to Tools -> Addons -> Themes, and click on "Get Themes" to download and install new themes. Remember that using hotkeys in Firefox to save time.