Yep. I just confirmed it… restarting Firefox seems to make the ID in https://demo.supercookie.me/ change. Thought as long as you keep Firefox session open the id will be the same.

I’m not sure if the links you reference are the whole story though, because they are talking about partitioning the cache per top-level domain, which I would expect wouldn’t have been enough, since the demo is specific to its top-level domain and it’s not necessarily about cross-domain id.