Google has added results previews for everyone! It looks great, you should try it out. You don't need my addon any more. Go here for more info:
This Firefox addon enables fast navigation of google results, while still retaining the google "feel". Navigate the results using the mousewheel, and mouse over a result to display it in a window to the right. Try it, it's fast and fun!
See the home page for further documentation and to submit feature requests.