F.F 91.12.0 ESR, No-Script & U-Block as plugins. JS white-listed for all domains on your page (except Google Tag-Manager). HTH.
It looks like F.F. doesn't support IndexedDB in private mode at all: https://bugzilla.mozilla.org/show_bug.cgi?id=781982, and IndexedDB is the main data storage for okso.app.
If this is the case I'll think about some fallbacks, maybe to LocalStorage, to make the app work in such cases as well...