Arronax has discontinued work on this theme, but has provided instructions for Firefox 4 and 5 on the following page:
Basically, you paste in the stylesheet provided at that link into the Stylish addon (or directly into userChrome.css), tell Firefox to turn off "Tabs on Top", customize the toolbar to "Use small icons", and then re-arrange the toolbar items as you see fit. With Stylish it's actually really quick to set up - definitely recommended. Just choose Stylish's "Blank style..." option so the theme changes show up on any URL.
Props to Arronax for such a brilliant theme. Still the best Mac theme, bar none.