Two main versions are displayed, one showing the tabs beneath the address bar and one with the tabs above it. The tabs-on-top look is nice and clean, and saves space, but eliminates the title bar.
Also on the wiki page is a demonstration of an idea for combining the go, refresh, and stop buttons into a single context-sensitive button.
As with the proposed changes to Firefox 3.7, the Firefox 4.0 changes are aiming to reduce interface complexity, increase page space, and hopefully increase clarity for the user while integrating more naturally into Windows. What do you think of the changes? Let us know in the comments.