This is probably the most useful feature in Firefox bookmarking system. If you have attached a keyword with any of the url, typing that and hitting enter in Firefox address bar will open it instantly.
It seems that keyword get more emphasis over the keywords of URL title tag ( the one which you see on the browsers top left) explaining why this happens. So if you have some of your favorite url which you open more often, just put a unique keyword with each of them and you would open them instantly. Via Firefox group on Stumbleupon