This question has been answered already multiple times on SO , here are a few links , that should get you sorted :
Custom CSS Scrollbar for Firefox.
Customize scrollbars using CSS
Customize scrollbars using CSS
How can I create custom scrollbar for Mozilla Firefox ith CSS?.
this should get you started. Long story short, a custom
css-styled div is used in conjunction with JavaScript to catch
click-and-drag events on the custom div. Wired up to these events are
methods that scroll the contents of whatever div the custom-scroller
has been attached to.
I'm all about the learning experience -- but after you've learned how
it works, I recommend using a library (of which there are many) to do
it. It's one of those "don't reinvent" things...
the above is a quote from this answer.
but if ur in a hurry and want to skip the learning phase , heres a plugin thats dead simple to use , and must less hassle then its competition .