O.K I've got a bit further with this and been able reproduce the problem more often. I've run 2 displays in extended mode for years with latest HP drivers. I have desktop links to FF on both screens. If I open several instances of FF on Monitor1, then close each and wait a few seconds before closing the last FF window, the desktop is left with a frozen portion of the last FF window. This clears when another app is overlaid or the screen is refreshed. If I repeat placing FF open windows on Monitor 2 and close them, there's no problem. I've confirmed there is no FF service running after the last window exits leaving behind a partial FF window overlaid on the desktop. If I re-configure for one monitor display, there's no problem. GFX drivers are up to date and it would seem there's a graphics issue, but nothing in my setup has changed? Just by chance I closed Kaspersky AV and couldn't reproduce the problem. So now I think Kaspersky AV is the culprit. I tried shutting down browser protection related options, but only shutting down the app itself stopped this fault. Since this doesn't occur opening several instances in Chrome, I can only think it's a conflict issue between Kaspersky AV and Firefox. Many AV apps try to do many things mostly in the background and some can't be disabled. I tried downgrading FF to older versions I've previously used without issues and get the same frozen screen with Kaspersky AV. So Kaspersky is what I now suspect to be part of the problem not Firefox. Firefox has enhanced security progressively over the many years I've used it. My wish list would be for basic AV file protection to be included in Firefox at a level already offered free in some AV apps, then it would be my one stop shop!