I think you have the wrong end of the stick here
Pelikal, the menu and tab bars still run horizontally, but the tab position is moved above or below the address bar.
about
:config is a feature of Mozilla applications which lists application settings (known as preferences) that are read from the profile files prefs.
js and user.
js, and from application defaults. Many of these preferences are not present in the Options or Preferences dialog. Using about
:config is one of several methods of modifying preferences and adding other "hidden" ones.
See:
About:config - MozillaZine Knowledge Base