This is a simple modification, with great results. Just like you can apply a theme in Windows, you can apply a theme to Mozilla Firefox. Follow these easy steps and within minutes you’ll have a new look for your browser.
First, go to Menu > Tools > Add-ons
A window will show, click on the Themes Tab and click Get Themes
You will be taken to the Mozilla Firefox theme page. Browse the themes, find one you like and click on the Install Now button. After a restart, you will have a fresh look for your browser.