Do you mean stop them from showing in the Bookmarks Bar, at the top of the browser window? You can either delete all of them in the Organise bookmarks… window under the Bookmarks menu, or make them visible only when you want them by toggling the Bookmarks Bar item under Toolbars> under the View menu. And, of course, when you add a bookmark, make sure you aren't adding it to the Bookmarks Bar unintentionally.
Hey Ray did you find a solution (or did anyone else). I just got v3 and when I clicked on the down arrow key at the location bar it gives me a bunch of past websites I visited. Try to look at the Firefox website but not that much help.