Here are a few firefox settings to make your URL field behave only as a URL field:

  user_pref("keyword.enabled", false);
  user_pref("browser.fixup.alternate.enabled", false);
  user_pref("browser.urlbar.suggest.searches", false);
  user_pref("browser.urlbar.filter.javascript", true);

You can put these in a "user.js" file in your profile directory if you don't have one already, or just set them in about:config