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?

This was due to the .git folder and some screenshots being included in the bundle. They no longer are. All of the code is open source and available on https://github.com/tom-james-watson/old-reddit-redirect.