I agree that issue with the scroll bar jumping all over is really annoying! It is actually a feature of Gnome and GTK. It can be changed by editing a file:
~/.config/gtk-3.0/settings.ini
[Settings] gtk-primary-button-warps-slider=0
I also find this behavior annoying. I tried adding this settings file (it didn't exist) and logging out and back in, but Firefox still jumps do the absolute location on the scrollbar. I'm on CentOS 7.3.