Fotiman - 12:03 am on Apr 2, 2013 (gmt 0)
Gave your example a try, captured a print screen of the hover, pasted it into a graphic program, and zoomed in. The color shown is not the exact color in the top part of the image, rather it looks there are actually 3 pixels affected. The top pixel looks like the top image color, while the next 2 are shades of the bottom color... it looks like Firefox is doing something like some kind of anti aliasing or something. I also tried making the background-position: 0 -25 and it's more obvious.
I suspect it has to do with the background-size, and some scaling that it's doing. But I'm not sure of the fix.