I have been using the Firefox 3 Beta since Beta 4, currently Beta 5. Probably the most notable new feature is the Address Bar, which not only searches the entire URL, but also the title and your bookmarks. It has taken me a little while to get used to this, but I am really enjoying this new feature.
This leads me to the point of this post. By default the address bar now only shows the top 12 addresses arranged by number of visits with the top results at the top. For me having just 12 items in the address bar is not enough for me, so I checked the preferences and I saw no way of changing this. Next, I checked the about:config page by typing about:config and hit enter.
(Warning: You can really screw up your Firefox install if you do not know what you are doing.)
The setting you are looking for to change the number of items in the address bar is browser.urlbar.maxRichResults. If you double click on this setting a dialog will pop up and you can change the number to anything you desire, I went with 25.