Firefox 3's URL bar has the ability to show rich results, relevant sites that you have visited in the past. Here's how to limit the amount of results you get.
1. Enter about:config in the URL bar
2. You might get a page warning against the changing of settings. Just click "I'll be careful, I promise!" to continue
3. In the filter, type browser.urlbar.maxRichResults
4. Double click the entry and change it to whatever amount you want (I have mine set at 10)