I had the same problem until today, when I started snooping. It’s really simple to reset it. All you have to do is type “about:config” into your address bar.
You’ll get to a page where it warns you if you make adjustments…click the button to continue, and then you’ll get a big list of random looking words, numbers…basically everything that makes Firefox what it is.
At the “Filter” box at the top, type in “keyword.URL”, one entry should come up. Double-click it, and it’ll prompt you to enter a string value. Just paste this link:
into that box, click OK, and you’re back to the default!