This does not work in Firefox. The squares get stretched, while the area you can access stays square, so that the lower row is hidden. This makes it impossible to play.
What happens is that the squares doesn't stay square like in your screenshot, but varies in size depending on which pieces are in that row. The height of a row varies, but the width of the columns always stays the same.
Edit: It is often not the lowest row that get hidden, often e.g row 4 and 5 get so low that you can't put any pieces there.