If -moz-border-radius is specified as pixels (50px), then all is fine:
But if the -moz-border-radius is given in percentage units (50%), then the background becomes square again:
Both boxes have a height and width of 100px with a padding of 50px and a thick solid border, a green background and yellow text.
Note also that the % seems to be relative to the total width, not the 'width' property as would be expected by CSS conventions.