Mozilla has been experimenting with new themes for Firefox 4 for some time. The new theme for Firefox is designed to modernize the web browser interface, reflecting the power, simplicity and customizability of the popular browser.
Stephen Horlander, a designer for the Firefox team, posted a few screenshots of a new design they are working on. The team has also posted a timeline outlining the various milestones and proposed changes.
New toolbar buttons, location bar and tabs for the Mac theme
Same on Windows
Other Theme Stuff