Rein, by Naoto Kusumi, is another clean, simple Firefox theme for those who appreciate invisible user interfaces while in use, and beautiful design when not.

The theme covers mostly everything: all built-in toolbar icons, sidebars, Add-ons Manager, Library, Options, Download Manager, all with a wink to good old Macintosh look. My only complain is that the buttonization of most icons in the managers results in some too small icons to be recognizable at a glance as they should be.

Get Rein at Mozilla Add-ons.