Featured in the lightest popular theme collection I did a few months back, the Elementary Theme for Firefox has grown on me.  Perhaps I like it because it is nice and simple?

The Elementary Theme is the official Firefox theme for the Elementary project.  Here is a little more about that project from the official web site:

The elementary project was started in January of 2008. It takes it roots from the Macesque icon set developed by Daniel Fore. In response to the ever popular oxygen and appeal projects for KDE, Fore sought to create something to unify the GTK desktop. Shortly after the 1.0 release of elementary icons, the project began to grow exponentially to incorporate themes for several window managers, a GTK 2 theme, a Firefox 2 theme, and a uSplash theme.

I am all for making things simpler, cleaner and clearer.  You can pick up the Elementary Theme for yourself on the Firefox Add-ons site.