Firefox starts split across two screens with titlebar inaccessible

When I close Firefox on a secondary monitor by just clicking the X to close, the next time I launch it, it starts with the window split across the secondary monitor and the primary, with the titlebar off screen to the top. I can then right click on the taskbar icon, close, then open again. It still starts split across the two screens but with the title bar aligned with the top of the screen. I can then drag the window to one screen.