Here is a nice article on Engineering the Firefox 3 User Experience. Some of the main points in the article:

• Usability is improved in the new version through visual consistency, with separate icons and themes depending on your operating system
• User feedback is a major factor during the beta tests
• Subtle usability enhancements, such as a new single-click bookmark system, improve the overall user experience

The Firefox 3 Beta can be downloaded here. I can’t wait to try it out.