Very deft and slick! I echo the other poster's call for this to be the default Ubuntu Firefox theme. For reasons I do not quite understand, The Linux "look" often comes off coarser than MacOS-X or WinVista. The only reason that makes sense to me is that I still know "linux geeks" who think GUIs are for pussies; they just don't think appearance is important. These aforementioned geeks don't want Linux to become too mainstream. But I think slick appearance is crucial to making Linux in general and Ubuntu in particular, a true alternative for those who are fed up with Microsoft but don't want to drink the Steve Jobs Koolaid either.
The only thing I would add is a slick new throbber. This is one area where Firefox has simply dropped the ball. Doesn't anyone do animated GIFs any more?
That one frustration aside, great work on a simple, but sophisticated and refined theme for Firefox/Ubuntu.