Some parts of Flash or storage info are still shared/leaked

[harl]

I had hoped that Priv8 would enable me to load the same Flash-based page in two different tabs (using separate login credentials), without either tab recognising the existence of the other (because I expected all Flash information was saved in the local storage thing), but it seems that some parts of one tab are still shared or leaked to the other, or simply accessed by the other tab somehow..

Contrary to that, I can do exactly what I want, by using certain different browsers:
These work in parallel: Firefox (or Nightly), Chrome, Opera, Vivaldi, Avant, SlimJet
and these don't: Midori, Kylo, Internet Explorer, Edge, Palemoon, Waterfox, SeaMonkey

It seems that the Chrome-based browsers are more separate in Flash matters than the FF-based browsers.
As soon as the Flash part of the page logs in (or whatever it does) from one of the browsers in the 2nd list (although obviously not all of them are FF-based), the Flash part in Firefox (etc) recognises the one in the other browser and logs me out, but most of the Chrome-based browsers have no issues of that kind.

Neither Icognito Mode (in Chrome), nor Private Tabs, Private Windows or Container Tabs (in FF) are separate enough from the normal tabs/windows to allow separate Flash logins.
And it seems neither is Priv8..
Is there anything you can do about this, or is it something that just isn't possible within Firefox..?