I'm currently on a high-resolution display (1920x1200 @ ~224 PPI), using Firefox 36.0.1 on Windows 8.1 with display (Windows DPI) scaling set to 150%.
This seems to cause favicons to not show up for some sites in the hot network questions list:
This seems to only affect those favicons more than ~4050px down the sprite image.
This occurs at default FF zoom. Zooming out one step in Firefox causes the missing favicons to appear. Zooming in has no effect.
These icons work correctly in IE, which leads me to believe it's a Firefox-specific bug. Perhaps it should be raised on their bugtracker, which I will look into in the morning, but since this affects SE specifically I'll leave this question here to see if anyone can reproduce, and maybe narrow down the cause or find a workaround.