See bug 1123299 comments 11-12, and testcase in attachment 8622718 [details]. The spin-box looks OK at the default font size, but its position shifts vertically as the font size changes. Giving input[type=number]::-moz-number-spin-box a fixed height of 16px (instead of max-height:1em) would make it stay in place nicely in vertical mode, but this prevents the control shrinking when used with a smaller font size in horizontal mode. So probably the additional wrapper suggested in bug 1123299#c12 will be a better option.