The answer is the Autocomplete Manager extension, which gives you a ton of better functionality such as partial matching against either the link or title of a bookmark or history entry.
You can search for a keyword in the URL itself, which will match against either history or bookmarks:
Or you can even search for a word in the title of the page, which I find particularly useful:
There’s loads of options, for instance you can turn off the matching anywhere or tell it to show bookmarks first in the completion:
Here’s the one oddity… this extension stores it’s own history database, so to clear the address bar history you’ll have to open up the History Manager tab, click the Retrieve button and then remove all. Even once you do that you still have to restart Firefox before the address bar will stop returning results.
I’m not the type of person that ever needs to clear my history, so this extension works quite well for me.
Download Autocomplete Manager from Mozilla Add-ons