what bothers me most is that they made the whole top part (including title, menu bar etc if you choose to enable it) transparent. I personally just find that horrible. Screenshot: http://0.tqn.com/d/browsers/1/0/Z/c/-/-/firefox-5-show-menu-...
Here is a screenshot comparing FF 3.5 to 4: http://cache.gawker.com/assets/images/4/2009/12/500x_firefox...
(3.5 is right)