Source

in ~/.mozilla/firefox/xxxxxxxx.default/chrome/userContent.css

input:not(.urlbar-input):not(.textbox-input):not(.form-control):not([type='checkbox']) {
    -moz-appearance: none !important;
    background-color: white;
    color: black;
}

#downloads-indicator-counter {
    color: white;
}

textarea {
    -moz-appearance: none !important;
    background-color: white;
    color: black;
}

select {
    -moz-appearance: none !important;
    background-color: white;
    color: black;
}