When using the Simple theme, Firefox selected menus are all-white (e.g. the menu title and the currently selected manu item). I'm not sure if this is a theme bug, or a Firefox bug, but switching themes fixes it. This is garnome 2.8.0, Firefox 1.0PR, Xorg 6.8.1.
I haven't tried it yet, but the patch at <https://bugzilla.mozilla.org/show_bug.cgi?id=251492> may fix this.