Flashing FF icon due to FVD opening a 2nd widow spontaneously

Win 10 O/S. Firefox v56.0.2 64 bit. FVD version 62.8.5. I've noticed

that when FF open, the icon in the tool bar flashes at times showing
that FVD has opened a 2nd window (not requested) of a saved page but
that when I click on the second window, this second window disappears.
Not a major problem but bit irritating.