Possibly there's a gtkrc file causing the colour scheme. Both Firefox and Rox are gtk+ apps. For firefox itself you could just install a theme of your choice if desired.
Or, you could get a gtk+ theme which will install a gtkrc file.
Or, you can place a .gtkrc-2.0 file (with the dot) in your home directory to override settings. Google for a sample .gtkrc-2.0 file (or take a gtkrc file from a gtk theme and rename it) and customize it.
P.S. I use a dark theme for xfce4 and it uses a gtkrc file. What I do is take and copy that file as ~/.gtkrc-2.0 so that my gtk+ apps all use that theme even when I'm not running xfce4.
These are the kinds of settings you'll find in a gtkrc file:
http://www.mikeserv.org/temp/gtkrc.txt