Mozilla appears to be planning to make some significant changes to the Firefox user interface in 3.7 of the browser.
The Mozilla site has some initial Firefox 3.7 mockups to start the discussion off among Firefox developers. Right now, they're showing only Windows changes, for both XP and Vista/Win7.
Simplification seems to be the order of the day. There's less button space on the left of the address bar, and buttons have a separated and distinct look to them. The whole File/Edit/View/etc. bar is gone, replaced by a View drop-down box to the right of the search bar, with a button to make the bookmarks bar appear. Far more of the window is 'glass', which makes it integrate better with Vista/Win7's OS elements like explorer windows and such.
The changes to the XP version are similar, though of course without the glass stuff. The main difference appears to be the Tools and Bookmark Bar buttons to the right of the search bar, which have a different design but presumably similar functionality to that in the Vista/Win7 design.
The changes look good to me. Removing the File/Edit/etc. bar, or at least hiding it away, makes sense. It increases the amount of your Firefox window that is used by the page itself, rather than the interface, which is one of the things I love about Chrome. It seems like use of the Home button has fallen out of favour, so it makes sense to get rid of that, too. Of course, this is a mockup of proposed changes to a version 3.7, when 3.6 isn't even out yet. So it may look quite different by the time this version ships.