This has been driving me crazy for the last month. As you can see with the screenshot, there is a white border around Firefox. I have dual monitors, a 4K and a 1080P. Problem only occurs on the 4K, only displaying on the 4K monitor doesn't help. Running the latest version of Windows 10, and latest drivers for my GTX video card. I've tried refreshing Firefox, completely uninstalling, safe mode, disabling hardware acceleration. This problem does not occur every time I open Firefox, just the majority of the time. F11 to go full screen then bringing it back to a maximized window fixes it for that session. Firefox is the only program that has this issue. It really bugs me, haven't found anything that quite fits or fixes the problem online :)