Now go to https://rittervg.com/misc/ff/fpi.html On first load it should say the same. If it says the same timestamp that was stored on the first page - it's not working.
Source: I'm a Mozilla Developer who is one of the primary devs/supporters of First Party Isolation.
So wouldn't a better test be about a third party that was used in a first party context before? Since FPI goes beyond third party cookies.
Both tests are equally valid. I just gave one because trying to be exhaustive about testing it would be mind-numbing. The test I provded only does localstorage, but FPI also isolates DNS cache, H2, image cache, favicons, cookies, localstorage, indexdb, etc etc
You can do yours by visiting https://anonymity.is/misc/ff/fpi-iframe.html first; then visit the ritter.vg and rittervg.com links.
What surprises me the most is that not only Firefox but also my Safari Browser passes all those tests when ITP is enabled.