I have disabled the gfx scrollbar in Preferences->Debug to have a somewhat more consistent look and feel. However, Mozilla's sidebar will always use the graphics scrollbar.
spam: changing qa contact on sidebar bugs from paulmac to email@example.com (all 67 of them!)
Are you talking about the sidebar tabs, or do you mean the customize dialog? The sidebar tabs use iframes (so it's up to the iframes to show scrollbars). The sidebar customize dialog uses trees which can only use gfx scrollbars.