I like the overlay scrollbars that come with natty beta 2 by default! How do I enable them on fx4?
That is currently not possible, due to the libraries that Firefox's user interface is written using.
There is a bug report that you can mark as affecting you: https://bugs.launchpad.net/firefox/+bug/767040
If the intent is to improve contrast for the existing Firefox scrollbar (not the overlay), a workaround is available in the form of an add-on which has been reviewed here and is available via from the official Firefox add-ons page. To quote from there: "This add-on is mainly for people, who want different "high contrast" scrollbars for their default Firefox theme. These scrollbars can be more kindly for eyes".
I don't use that add-on since I already have the Stylish add-on installed: this add-on allows one to modify scrollbars. My simplistic tweak is in the comments section of the first link.
This works well in 11.10. Try it in 11.04.
I've tried it in 12.04; it does change the size, but not the color.