Border around content is too wide (changes with resizing)
On (maximized) startup, the border surrounding a page's content is too large (image Startup.jpg).
The issue is easy to see when Firefox is resized. The border for an 898 pixel wide window (898.jpg) is very large, while enlarging the window one pixel (899.jpg) results in a small or non-existent border.
Windows 7 Firefox 101.0.1 (64-bit) Screen 800x600 at 125% magnification (I'm partially sighted)