Thanks! Had just planned to answer my question myself, but you were faster. Anyway, here is my code:

/* Set white (transparent) background color for dropdown selection boxes */ select {    -moz-appearance: none;    border: 1px solid grey;    border-radius: 4px;    padding: 2px 19px 2px 1px;    background-image: url("chrome://global/skin/icons/arrow-down-12.svg");    background-position: right 4px top 50%;    background-repeat: no-repeat;}/* Optional highlight while hovering (light grey) */select:hover {    -moz-appearance: none;    border: 1px solid grey;    border-radius: 4px;    padding: 2px 19px 2px 1px;    background-color: #e9e9ed;    background-image: url("chrome://global/skin/icons/arrow-down-12.svg");    background-position: right 4px top 50%;    background-repeat: no-repeat;}/* Optionally set white background color for dropdown options list */option {    background-color: white;}