Firefox UI is as busy as you make it[1]. It's very customizable without much effort. Not that it couldn't use a refresh but I quite like it the way it is.
EDIT: The commenters below get it right. Tree Style Tab[2] add-on is responsible for the tabs and you can remove the clutter by right-clicking the refresh button for example, choosing "customize..." from the context menu and just dragging all the unused stuff away into the box (a window that will appear). Toolbars can be hidden from View → Toolbars. Enabling integration with Ubuntu also thins it a bit.
[1] http://i.imgur.com/zdIy63Q.png
[2] https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...