This is the upper right corner of the Firefox window. It shows that the border is unusual...

Some interface drawing problems have been noted. A suggestion one user found helpful was from Reddit:

I had the exact problem and I solved it by going to Settings>personalisation>colours>"Show colour on Start, taskbar, action centre and title bar" and changing "On" to "Off".

Link from: Firefox window doesn't display correctly in Windows 10 Home since Firefox Quantum

About resizing the window:

If the grab handle is not there, try using the old keyboard method:

• press Alt+Spacebar to open the control menu
• click Size or tap S to start size mode (if it's grayed, click Restore first)
• to move the right side of the window, press the right arrow key to select it, then press right arrow repeatedly to expand in that direction or left arrow repeatedly to move that side of the window leftward; finish by pressing the Enter key
• to move the bottom side of the window, press the down arrow key to select it, then press down arrow repeatedly to expand in that direction or up arrow repeatedly to move that side of the window upward; finish by pressing the Enter key

re: press Alt+Spacebar to open the control menu

I just tried it, but both Restore and Size are grayed out.

edit: after following the 'personalization' hint, both the dragging and alt/space worked.

Thanks for the personalization hint: now the window can be re-sized.