Firefox 1.5 and the Greasemonkey extension. You will need to install the extension before the script will work.
Once the script has been saved to your computer, open it in Firefox by dragging and dropping the actual file into the browser window, and then click the Install button that appears. If you don't see the button go to Tools > Install This User Script. Reload any open Google pages and you will see the script in action.
If you have a fresh install of Firefox 1.5 and the script doesn't work with Greasemonkey installed, try closing the browser and then opening it up again. It should now work.