How to Enable Overlay Scrollbars in Firefox on Ubuntu 11.04

11.04firefoxoverlay-scrollbars

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

In 12.04 LTS there's a slightly cleaner way to do it. Just run

gsettings set org.gnome.desktop.interface ubuntu-overlay-scrollbars false

in a terminal. Apps opened afterwards will have the non-ayatana scrollbar.

To get the default behavior back, run

gsettings reset org.gnome.desktop.interface ubuntu-overlay-scrollbars

If you are using 12.10 or later, run

gsettings set com.canonical.desktop.interface scrollbar-mode normal

And to return to the default, run

gsettings reset com.canonical.desktop.interface scrollbar-mode

Make sure you have the packages overlay-scrollbar and liboverlay-scrollbar-0.1-0 installed. (It's possible that you somehow lost the package ubuntu-desktop which usually is responsible for pulling in such new packages.)
Overlay scrollbars only work in a handful of applications right now (easiest to test are probably Nautilus and gedit), so maybe you have just tested the wrong applications.