From MozillaZine Knowledge Base
Scrollbars in the user interface may appear on the left or right side of the content, depending on whether the language viewed is read right-to-left or left-to-right. However, a user may want to keep the scrollbar on one side irrespective of the contentâ€™s language, or even the host OSâ€™s language. This preference is an attempt to make the scrollbar position as customizable to the end user as possible.
Possible values and their effects
Place the scrollbar at the end (right in LTR languages, left in RTL) of the content, based on the UIâ€™s direction. (Default)
Place the scrollbar at the end (right in LTR languages, left in RTL) of the content, based on the document/contentâ€™s direction.
Place the scrollbar on the right side of the content.
Place the scrollbar on the left side of the content.
First checked in
Has an effect in