Download UltimaDark - The Fastest Dark Mode Extension for Firefox. UltimaDark uses agressive & smart techniques to turn even the sunniest websites into realms of darkness.
Although it works well, this is so experimental, it makes lab rats look like seasoned professionals.
Go ahead, embrace the shadows! 🦇
I just tried it on a Firefox fork and it works fine.
Just go to the link in OP, then enable Desktop Mode in Firefox. That will let you install it.