Gray Box in Top Left Corner

Hello,

Recently I have had a problem with Mozilla Firefox where a partially transparent gray/black box appears in the top left corner of my computer monitor. I am using a Toshiba Satellite L755 laptop (1366x768 screen) attached to an HP S2031 external monitor(1600x900 screen) via a VGA cable. I am using Windows 10 and the monitor is set to "extend" mode, where I can drag windows from one monitor to the other. When a Firefox window is on the laptop's screen there is no gray box, but if I move the Firefox window to my external monitor and increase the width of the screen past 1366 pixels, the box appears with a width of (Firefox window size - 1366). The rectangular box also extends to the bottom of the bookmark bar. I have only noticed this effect when stretching the window horizontally, not vertically. I have tried disabling hardware acceleration, clearing cookies and cache, starting Firefox in safe mode, and reinstalling Firefox, and none of these helped. I do not know exactly when this issue started occurring but I believe it was a couple updates ago. I also posted to the Firefox subreddit but it seems no one was able to help me. I would really appreciate any help or ideas as to how this can be fixed. Thanks a lot for your time.