Suggestion: Private mode simultaneously to non-private-windows

If you open a new window in private mode, the existing windows are
closed (and restored again, if you close the private mode). It would
be a much better usability, if existing windows are not closed and
they could be used simultaneously to private-mode windows. Just like
in Google Chrome. Because maybe you need the private mode only on one
site and want to work on with the other sites.