Go into your FF profile, open SiteSecurityServiceState.txt - it will show every HSTS entry separated into firstparty domains.

Similar, in the storage folder in your FF profile you can see that every firstparty website has it's own folder and third party cookies are places inside that folder and can not share data with other folders.

Not super reliably. But try this one: https://github.com/mozfreddyb/test-firstpartyisolation

I won't copy paste my reply from above, but rather link it: https://news.ycombinator.com/item?id=17949613