Being installed in a Linux distro, Firefox is subject to the system-wide theming prevalent in that environment. I am also currently annoyed with both the thinness and lightness of scrollbar controls, but the problem exists more or less the same across applications. You may have some control of this in your desktop environment or window manager themes or settings. You may wish to check your distro's forum. If not, i am not sure what you have to do to get your distro and DE to ignore the system theme for native UI elements of an application like Firefox, but if you can, you would have to edit images and/or a text file somewhere like: /chrome/browser/skin/classic/browser/ in /usr/lib/firefox/browser/omni.ja I haven't kept up on this stuff since Mozilla murdered theming, not that i was ever an expert.