Does anyone know of a Firefox fork that supports AMOLED darkmode?

Try this. You can add any add-on to Firefox on Android by using your own custom add-on collection. It works great!

Since this community is for Android

I assume you want AMOLED dark in Firefox Android

Sadly you cannot apply add-on themes on Firefox Android

Although you can use add-on like dark reader to make website AMOLED dark

On Android, it doesn't appear to be possible yet - https://connect.mozilla.org/t5/ideas/provide-a-black-dark-mode-in-firefox-mobile/idi-p/2578/page/2#comments

Maybe look in themes section, located bottom left addons page in Firefox desktop.