It is definitely still meant to be working on firefox android. It's working fine for me and I've had no other reports of it not working. I'd open an issue on the github repository (https://github.com/tom-james-watson/old-reddit-redirect/issues) if you think there's a bug.

Also, until firefox android allows for installing of arbitrary extensions, you need to go through this awkward flow to add the extension: https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly. I assume that's what you're doing?