Today, Stephen Horlander, the visual designer of the Firefox team, provided some latest screenshots of the Firefox 4.0 user interface again. He said that one of the development focuses of Firefox 4 is to provide more secondary user interfaces for the browser, also known as in-content UI, which has already been applied in the add-ons manager.
In addition, parts that adopt the in-content UI also include Restore Session, about:config, network error pages, and phishing/malware warnings and pages. In the future, this secondary user interface will also be integrated into preferences, library, home tab, tabcandy, and about:firefox.