Work on the new linux theme for Firefox 3 is progressing nicely due to a very active group of contributers. To match the operating system’s theme, Firefox’s navigation toolbar will use native GTK icons. The remaining icons are being drawn in the Tango style. I owe a huge thanks to Andreas Nilsson, Michael Ventnor, Kalle Persson, Lapo Calamandrei, Garrett LeSage, Michael Monreal, jimmac, and everyone else who is contributing to help ensure that Firefox 3 looks great on linux.
[Update: there is some very thorough coverage over on Ars Technica with lots of screenshots]