To get this extension to work on Windows you need to find an old version of iCloud and turn of autoupdate. I'm using 7.21, and the extension works for me.

I think what's important is that you don't install iCloud via Microsoft Store because that means the app is installed in a different location from what the extension expects

Cannot get it to work. iCloud keeps asking to download the extension while Firefox states it is installed and active.

don't do it. Apple doesn't believe bugs exist. Everything it's doing wrong, is a feature.