It happens to me on Linux. I have GTK and QT configured with dark themes. To overcome this, I have to configure Firefox with a default styling in ~/.mozilla/firefox/*/chrome/userContent.css:

    input, button {
        -moz-appearance: none !important;
        background-color: white;
        color: black;
    }

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

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

Now that I look at it, I wonder why I didn't join all 3. Whatever.