Smart Location Bar (aka "Awesome Bar") learns as you use Firefox. Type in a keyword and the Awesome Bar searches through your history and bookmarks and presents a list of recently accessed pages with that keyword. It works even if you know the URL. Some people do find it annoying
, so there is a way to disable it (a small tweak to your about:config file). An Add-On allows you to tweak which of the search results you want to always be on top.