What Firefox extensions are you using? Check out Multi Account Containers on Firefox if you haven't already. It fulfills all my privacy requirements when paired with uBlock Origin and Privacy Badger. https://addons.mozilla.org/en-US/firefox/addon/multi-account...
I tried this the other day because Google Stackdriver claims to need third-party cookies enabled, which is an absolute no-go, but it also refuses to work in incognito mode and even the container mode didn't help, IIRC.