Hi there!
It's a great theme but is there any way to get also those popup-sliders into Firefox?
You know - those two arrows that appear when you move your mouse over the slider.
http://i.imgur.com/hnGi4RU.png

The ff.rc included works fine for me so long as you actually follow the instructions (putting the scrollbars directory in /usr/share/themes/Radiance/gtk-2.0/, not in the apps subdir!

I did need to install gtk2-engines-pixbuf to get it to work though - I'm running Ubuntu 12.04.

I followed the instructions provided, but now I do not have any scrollbars in Firefox, and when I start any other apps which rely on ff.rc, I get errors regarding not being able to locate image files in pixmap_path and background image options specified without filename. The files are there, just not loading properly.

OK. I figured out why. OP doesn't have the paths in his ff.rc file correct. The paths are supposed to be relative to the location of the ff.rc file. For example, OP has "../scrollbars/trough-scrollbar-horiz.png", which should just be "scrollbars/trough-scrollbar-horiz.png".

Note: The "../" needs to be removed. The ff.rc file is in the same directory as the scrollbars folder. Easy fix. Just do the following:

sudo gedit /usr/share/themes/Ambiance/gtk-2.0/apps/ff.rc

Then CTRL+h and search for ../ and replace it with nothing.

Click close, then CTRL+s, CTRL+q.

Now it should work as OP intended.