From MozillaZine Knowledge Base
In Mac OS X, the behavior of clicking inside the scrollbar is determined by a system preference (“Click in the scrollbar to:”). In GTK the setting is called gtk-primary-button-warps-slider.
If ui.scrollToClick preference is not set, the system preference is read; otherwise, this preference’s setting takes precedence.
Possible values and their effects
Clicking on a scrollbar (not dragging the scrollbar’s “thumb”) will scroll to the view corresponding to the clicked point.
Clicking on a scrollbar will increment the view up or down by a page, unless using the middle mouse button or holding Shift (or the Option key in OS X) (in which case the view will jump directly to the position corresponding to the clicked point). (Default on Windows)
First checked in
Has an effect in