Utopia White has a lot of potential, but the current version (3.10) just misses the mark. The ice white and coal black visual elements provide good contrast unlike the default Firefox theme, but there are several key areas of Utopia that lack refinement.
For example, the vertical scrollbar is a very dark, inky black pattern on a white background, yet the tabs -- arguably the most important visual feature of any browser since you have to read the tiny text within them -- are grey text on a pale grey background. Whoops! Not good. Grey text on grey backgrounds is very hard to read -- I wish every themer would get this simple fact through their head -- and certainly has no place in a theme called "Utopia White".
Another head scratcher is the Bookmarks Toolbar: I don't know about you, but I have folders and text on mine; it's the last place I want to see thin horizontal lines in the background. Text + background stripes = visual clutter. Please don't do that.
Also, the black spaces between tabs is too large and the feathered edges on the buttons looks sloppy.
Overall, I like this theme very much. The Bookmarks, Tools, and other menus are well-laid out and easy on the eyes. If those few faults mentioned earlier were fixed, this would be a 5 star theme.
[Tested on a MacPro running Leopard 10.5.5 and Firefox 3.1b1]