On Firefox for Ubuntu, just using
input[type='number'] {
-moz-appearance:textfield;
}
did the trick for me.
Adding
input::-webkit-outer-spin-button,
input::-webkit-inner-spin-button {
-webkit-appearance: none;
}
Would lead me to
Unknown pseudo-class or pseudo-element ‘-webkit-outer-spin-button’. Ruleset ignored due to bad selector.
everytime I tried. Same for the inner spin button.