BB 32411: Letterboxing, exempt view-source: URIs.

      contentPrincipal.isSystemPrincipal ||

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

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

      // ... source code

      currentURI.schemeIs("view-source") ||

      // ... browser extensions

      contentPrincipal.addonPolicy

    );