Previewing a persona only works on sites that are whitelisted via an allow exception. You can see the permissions for the domain in the current tab in: * Tools > Page Info > Permissions > [X] Install Extensions or Themes * Tools > Options > Security : "Warn me when sites try to install add-ons": Exceptions Make sure that you do not use [[Clear Recent History]] to clear the 'Site Preferences'. See http://kb.mozillazine.org/Themes#Lightweight_theme