CSS is what controls that. What I would do is, I would google for "CSS scrollbars" and use the links I find off that
This only works in IE based/related browsers (I think that means Opera too).

The truth is that CSS was never ever meant to control things such as scrollbar since that is actually a part of the OS's implementation of a scrollbar so if the user is impaired in some way, the colours you choose to colour your scrollbars could affect him in a bad way. Such a user would have themed the scrollbar in such a way that it optimizes browsing for him and you controlling his scrollbars from your CSS could impair him.

Now for the IE part, to put it loosely, it was Microsoft that introduced this scrollbar colouring thing.
This probably hapenned during the browser wars to get the edge over its competitors but I'm just guessing there (I hate reading anything other than code off the computer and I don't watch the news ).

So don't think that it's because of Firefox that you can't colour your scrollbars

Hope that made sense