When you are typing in a URL in Firefox 3, it pops up a rather large box of search results for previously visited URLs that match what you have entered so far. It's not just the URL as in previous versions of Firefox, it contains more information about the site. They refer to this as the "Rich Results".

If you want fewer of these results, you can set that using a new configuration parameter. Just go to the address bar and type about:config and confirm any warnings you get. Then scroll down to the setting called browser.urlbar.maxRichResults and double click on the entry. You can then enter the maximum number of results that you want to be shown.