It's only been a few weeks since Firefox 3.5 went live, and Mozilla has already released a mockup of 3.7's theme. They have posted several screenshots of a potential user interface for the future build. The theme is displayed on Windows Vista and 7 (with and without Aero), as well as XP.
While it's still in the workshop, the images provide a glimpse of some changes you might see in the coming versions of Firefox. In addition to embracing glass on Windows Vista and 7, the future layout may include a tools button on the right side of your search bar, and a page button on the left of your tabs.
Mozilla hopes to introduce a more significant series of changes with Firefox version 4.0. Among the current ideas is one which would combine and move the reload, stop and go buttons, the addition of a tab sidebar, moving tabs above the main toolbar and quite a few other UI tweaks.