The Firefox 4.0 page on the Mozilla wiki is at pains to stress that the mockups are not final designs, although they do give an indication of some interesting features which could be just around the corner.
Two main versions are displayed, one showing browser window tabs beneath the address bar and one the other way around.
As with the proposed changes to Firefox 3.7 revealed last week, the Firefox 4.0 tweaks appear to be designed to reduce interface complexity, increase page space, and integrate more naturally into Windows.
Also being explored is the possibility of having a combined go/refresh button. "It will help the users who are just switching from IE," says Mozilla's wiki.