I came across an article on the Null Set about Making Anime Personas for Firefox, with some excellent examples (that set is listed under Steelbound). I also found rather nice collections by Songbird, Bellas, Kawaii Mooru, and Fleur, to name just a few. I also found huge selections of other science fiction themes, but these should give you the idea.

Building your own seems pretty simple and straightforward; the short version is you create an image each for the top and the bottom of the browser, both 3000 pixels wide, with the top at 200 pixels tall and the bottom 100 pixels. For slightly more detailed information, check the Persona Create and Test page. These are too much fun; I think the trick is building ones that look good but don’t interfere too much with the tool bars. If you need to install the personas Add-on, you will find it here.