Turns out Firefox doesn’t work well with a lot of things. :/

:-moz-placeholder { /* Firefox 18- */
  color: #aaa;
  font-size: 18px;
}

::-moz-placeholder { /* Firefox 19+ */
  color: #aaa;
  font-size: 18px;
}

These are for the others:

:-ms-input-placeholder { /* Internet Explorer */
  color: #aaa;
  font-size: 18px;
}

::-webkit-input-placeholder { /* Chrome, Safari */
  color: #aaa;
  font-size: 18px;
}