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.

crap

The addon is completely broken (and has been so for years now!). The iCloud app acts like it's not installed when attempting to use it to sync bookmarks, even though it's installed and enabled. Fix your software, Apple!