The problem is that the top margin of the H1 element is non-zero, but the padding of the containing "c4" div is 0. In this case FF leaves space above the containing div to make room for that H1's top margin. I don't know if this is a feature or a bug, but there are two ways to fix it: (1) give the H1 element a margin-top value of 0, or (2) give the .c4 div a padding-top value of 1px (or more).
