While messing around with a couple of Firefox themes, I discovered something about the Firefox add-ons dialog. When you select the default theme, it will picture the default theme that comes with Ubuntu (Tango in the case of Intrepid Ibex).
However, when you restart with the “default” theme, it will display the desktop theme, including icons. While I figured this is how it would behave, this behavior may be confusing to some users.
Dances With Ubuntu 