Firefox will not resize with default theme
Firefox will not resize, or snap using the Windows key, when the default theme is selected. The behaviour is complex. - With other themes selected it works fine (thanks to this question for that solution https://support.mozilla.org/en-US/questions/1184616). - The left, right and bottom borders of the window are not being drawn. They initially show up white but sections of other applications end up 'trapped' in them. - Sometimes resizing one of the sides of the window with the mouse will work, but mostly they do not. - Maximising the window with the titlebar button or Alt+Up works. - Snapping the window either by dragging to the top or sides of the screen, or by using Alt+Left and Alt+Right does not work.
I an running Firefox 60.0.1(64-bit) on Windows 10. I am also using 200% scaling on my default display and 150% on my second display.