Social Fixer is the most powerful utility to adjust the behavior of facebook. It uses a browser plugin which provides a javascript library to change the css and javascript provided by facebook.
The plugin is available for: Mozilla Firefox, Google Chrome, Rockmelt, Opera, Safari, Greasemonkey
It is not available for Internet Explorer.
The first step is to download Social Fixer here: http://socialfixer.com/
It is important to choice the right download depending on the used browser.
After downloading the plugin can be installed by executing the downloaded application.
It might be necessary to restart the browser after installing the plugin. So make sure to save important work done in the browser.
Once the installation is finished and you go to facebook the first time a wizard is shown which helps you with the most popular enhancements and changes.
For a lot more possible enhancements it it possible to get the Social Fixer configuration menu by clicking on the new wrench symbol next to the profile link on the top right corner of the facebook webpage.
Make sure to update Social Fixer regularly since facebook is making a lot of changes and the plugin might break.