I prefer Dark Background and Light Text by Mikhail Khvoinitsky: https://addons.mozilla.org/firefox/addon/dark-background-light-text/.
You should try it. There are some obvious differences. And then some:
- It works with internal Firefox pages.
- It works with Windows High Contrast theme.
- It doesn't break any add-on pop-up panel.
- On facebook, it preserves original site icons.