As explained above, it is some kind of graphic glitch. It was also present when I installed FF4 some months ago so I reverted to 3.6. It's still present, and it also occurs on my other computer which has a 185 Nvidia card from memory. The card drivers are the latest available. The display monitors are different brands so it's not the monitor I guess. For the record, I did turn hardware alleleration off after first finding the problem. The issue went away. I have turned hardware acceleration back on, and no problems again so far this session.