When a container's contents which are larger that the size of the container, scroll bars may be placed at the side of the container to allow the user to scroll around in the container. The scroll bar may also be used independently when a numeric value or percentage needs to be selected by the user. The user can adjust the position of the scroll bar by clicking arrows on either end of the scroll bar or by dragging the scroll bar thumb around.
More information is available in the XUL tutorial.
<scrollbar curpos="5" maxpos="50"/>
valueattribute changes when the tray of the scroll bar (the area in which the scroll bar thumb moves) is clicked, or when the page up or page down keys are pressed. The default value is 10.