Fix dark input Firefox

style.css

input:not(.urlbar-input):not(.textbox-input):not(.form-control):not([type='checkbox'])
       :not(#search_form_input):not(#search_form_input_clear):not(#search_button):not(#search_form_input_homepage) {
    -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;
}

Select arrow seem be hidden, to fix that replace

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

by

select {
    -webkit-appearance: none;
    -moz-appearance: none;
    background: transparent;
    background-image: url("data:image/svg+xml;utf8,<svg fill='black' height='24' viewBox='0 0 24 24' width='24' xmlns='http://www.w3.org/2000/svg'><path d='M7 10l5 5 5-5z'/><path d='M0 0h24v24H0z' fill='none'/></svg>");
    background-repeat: no-repeat;
    background-position-x: 100%;
    background-position-y: -2px;
    border: 1px solid #dfdfdf;
    margin-right: 2rem;
    padding-right: 2rem;
}

Also you can replace background-color: white to background-color: transparent which is more convenient

Here my fork