issue page: vote icons are overdimensional big in firefox below v57.x

A (more legacy) Firefox (of a version below 57.x) can not display the "vote-thumbnail-icons" correctly.
They are overdimensional big (see attached screenshot).
It can be a problem of the implemented icon-set or some CSS-settings inside the frontend-page.

(tested with Firefox 52.9.0 (64-Bit) and with Firefox 57.x ESR)