Keep in mind that browser-specific extensions that don't affect the functionality of the page really aren't going to matter. If you want IE users to have colored scroll-bars, even if Mozilla users have to go without, it's not going to break the page or anything; browsers that don't support the extension will just ignore the style rule.

In some cases, other browsers may even eventually support the extension, whether its part of the official CSS spec or not. This was exactly the case with Iframes, for instance, which were originally an IE-only extension.

dmeister