Firefox on linux with a dark theme has black input fields

If you are using firefox on linux with a dark theme, unstyled input fields will have black background and white text. Sometimes just white text on white background.

According to this bug report it has been this way for the last 17 years. To fix it, pop by about:config and add a new key-value (String): "widget.content.gtk-theme-override": "Adwaita:light". After restarting firefox, things will be as they should be.