How to Make Firefox 4 Look Like Firefox 3
 

After upgrading Firefox 3 to version 4, when you first launch the new release the interface is drastically different. If you find you are more partial to the Firefox 3 interface, with a few configuration changes, you can easily make Firefox 4 look like its predecessor.

http://www.howtogeek.com/58035/how-t...ike-firefox-3/