Bug 31064: Letterboxing, exempt browser extensions.

      // ... privileged pages

      contentPrincipal.isSystemPrincipal ||

      // ... about: URIs EXCEPT about:blank

      (currentURI.schemeIs("about") && currentURI.filePath !== "blank")

      (currentURI.schemeIs("about") && currentURI.filePath !== "blank") ||

      // ... browser extensions

      contentPrincipal.addonPolicy

    );

  }