Aside from the default Firefox theme, there aren't any really cool themes that I've found -- but I do like a couple of themes which give Firefox the look of MS Office 2003. Here are the links:
Outlook 2003 Blue (https://addons.mozilla.org/en-US/firefox/addon/890)
Outlook 2003 Green (https://addons.mozilla.org/en-US/firefox/addon/892)
Outlook 2003 Silver (https://addons.mozilla.org/en-US/firefox/addon/894)
An ideal theme for me would be a really minimalist theme which uses very dark colours, but doesn't overdo it. Yup, it's a high hurdle.
Update: Just found the perfect theme: Miint (http://www.twistermc.com/blog/miint/). Mmm, tasty.