As user thirtydot explained in another question, there is no way to customize scrollbars in Firefox as its possible in Chrome.
Also, there is no way to actually “force” Firefox render the old-style scrollbar since the default scrollbar used in the system is predefined by the OS itself (note that you can modify which scrollbar you want in System Preferences).
In other words, until Firefox supports native custom scrollbars, it is only possible with JavaScript plugins such as jScrollPane and similar.