I have two monitors with resolution of 1920x1080 - my laptop screen and a 24" Dell. I'm using Windows 10 (updated) and the latest FF version (currently 57.0.2 64bit). My laptop screen is the main window and the Dell is the extended one. When my FF window is on the laptop screen (even when both monitors are connected) everything is fine. When my FF window is on the Dell screen, when I click on an addon icon on the toolbar, the opened window is cut on the bottom (and maybe on the sides as well), and since there are buttons on the bottom usually, I cannot use the addon properly. No scroll is available and I found virtually no way to get to the bottom of the window.