Walnut for a wooden look and feel. Walnut is a complete skin redesign so that all windows, widgets, panels, and many extensions have a wooden look and feel.
Install the About Home Themer extension to also theme the about:home page.
GNOME 3 theme for Firefox — Previously known as "Adwaita"
It is based on original firefox 4.0 Mac OS theme. MacOSX theme for firefox 3.x is at https://addons.mozilla.org/en-US/firefox/addon/macosx-theme/
New version optimized for the new Australis interface.
Theme "Pinguin" composed of large icons for easy navigation.
Theme based on the Nuvola icon set by David Vignoni (http://icon-king.com/?p=15).