as most of you know, theme support is working its way back into the latest mozilla releases (9.1, 9.2+). so far, the sky pilot theme is very good looking and you can install it by View/Apply Themes/Get New Themes. the aqua theme also looks great, but in order to install it, you need to go to the developers site and pick the latest release. http://www.simweb.net/eric/projects/Aqua/ the developer hasn't updated the x.themes.org site so don't install from there.
