Well, I can't tell you why that happened, but as Rollo suggests, there are better ways of remembering a site, such as adding it to your favourites.
If I want to make a temporary note of a site/page I create a short cut on the desktop, and delete it after it's no longer needed.
Firefox is great for this, as you need only click on the tab and drag it to the desktop. Of course you can't do that if you've got Firefox maximized, unless, like me, you have two or more monitors. But believe it or not, you can also drag a Firefox tab to your Quick Launch toolbar.