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UltimaDark - The Fastest Dark Mode Extension
(addons.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
It's been in development for 4 years.
That doesn't mean it's stable. From his own description:
Yea, I mean it will take eternity(not really) to become stable. xD
Because of the fact that UltimaDark is going the hardest route, using a totally different API, unlike Dark Reader