I'm out of ideas on how to fix this, every now and then my Firefox browser gets a random white border around it. Windows 11, no compatibility mode active for the application.

When I maximize and minimize the window it goes away for a while.

One possible hint: This has started to show up after adding a third display (a 4K TV) to my setup. Even if the TV is fully off. But the TV is the only display which has a different resolution scaling active (all other displays are at 100%).