I can barely see the Firefox scroll bar. I believe this is controlled by the operating system. I looked in Ubuntu Tweak > Miscellaneous but I don't see any color or contrast settings there.
How can I get a high contrast scroll bar?
Take a look in /etc/X11/app-defaults. Many of the files have scrollbar colors set for applications. e.g. for the XTerm-color:
*VT100.scrollbar.background: gray60 *VT100.scrollbar.foreground: rgb:a/5/5
Change the VT100 to Firefox, change the colors to what you want, and put the lines into a new file /etc/X11/app-defaults/Firefox Next time you start X, the new defaults should be present. (Haven't actually tried this myself).