[tedunangst]

Obvious solution is obvious. If the user is using Firefox, instead of replacing the scroll bars, replace the page with a message telling them to switch to a better browser. That way everybody wins and it's so much less work.

[ergothus]

Or more obviously - if a browser works a certain way, and the user has chosen to use it, let it be. If the designer doesn't like it, well - the user chose to use this browser, and they choose to use your site. If you start trying to change one of their choices, they might change the other.